Adopt for new multi-threaded cursive capabilities

[azat]

Note: this includes #40

In 1 cursive now requires Send + Sync, and the reason for this is:

The View now requires Send + Sync, to allow accessing or moving views between threads.
This prevents using Rc/RefCell, and may require using Arc/Mutex instead.
This should eventually open the way for more multi-threaded processing of the view tree.

[correabuscar]

This PR passes cargo build and cargo test with cursive-core = "0.3.5" and cursive = "0.20.0" and then I've tested grin (latest master) which passes too.

Would be nice to have it as version 0.15 maybe?

If so, it would allow grin to use an updated cursive and avoid this error:

   Compiling grin v5.3.0-alpha.1 (/home/user/1tmp/grin)
error[E0599]: the method `with_name` exists for struct `TableView<WorkerStats, StratumWorkerColumn>`, but its trait bounds were not satisfied
   --> /home/user/1tmp/grin/src/bin/tui/mining.rs:238:31
    |
238 |                 Dialog::around(table_view.with_name(TABLE_MINING_STATUS).min_size((50, 20)))
    |                                           ^^^^^^^^^ method cannot be called on `TableView<WorkerStats, StratumWorkerColumn>` due to unsatisfied trait bounds
    |
   ::: /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cursive_table_view-0.14.0/src/lib.rs:117:1
    |
117 | pub struct TableView<T, H> {
    | --------------------------
    | |
    | doesn't satisfy `_: Nameable`
    | doesn't satisfy `_: View`
    |
    = note: the following trait bounds were not satisfied:
            `TableView<WorkerStats, StratumWorkerColumn>: cursive::View`
            which is required by `TableView<WorkerStats, StratumWorkerColumn>: Nameable`
            `&TableView<WorkerStats, StratumWorkerColumn>: cursive::View`
            which is required by `&TableView<WorkerStats, StratumWorkerColumn>: Nameable`
            `&mut TableView<WorkerStats, StratumWorkerColumn>: cursive::View`
            which is required by `&mut TableView<WorkerStats, StratumWorkerColumn>: Nameable`
    = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
    |
17  + use cursive_core::view::nameable::Nameable;
    |

and an updated cursive-core to avoid this error:

   Compiling cursive_table_view v0.14.0
error[E0277]: `Rc<(dyn for<'a> Fn(&'a mut Cursive, H, std::cmp::Ordering) + 'static)>` cannot be sent between threads safely
   --> /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cursive_table_view-0.14.0/src/lib.rs:995:21
    |
995 | impl<T, H> View for TableView<T, H>
    |                     ^^^^^^^^^^^^^^^ `Rc<(dyn for<'a> Fn(&'a mut Cursive, H, std::cmp::Ordering) + 'static)>` cannot be sent between threads safely
    |
    = help: within `TableView<T, H>`, the trait `Send` is not implemented for `Rc<(dyn for<'a> Fn(&'a mut Cursive, H, std::cmp::Ordering) + 'static)>`
note: required because it appears within the type `Option<Rc<(dyn for<'a> Fn(&'a mut Cursive, H, std::cmp::Ordering) + 'static)>>`
   --> /usr/lib/rust/1.76.0/lib/rustlib/src/rust/library/core/src/option.rs:570:10
    |
570 | pub enum Option<T> {
    |          ^^^^^^
note: required because it appears within the type `TableView<T, H>`
   --> /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cursive_table_view-0.14.0/src/lib.rs:117:12
    |
117 | pub struct TableView<T, H> {
    |            ^^^^^^^^^
note: required by a bound in `cursive::View`
   --> /home/user/1tmp/cursive/cursive-core/src/view/view_trait.rs:34:33
    |
34  | pub trait View: Any + AnyView + Send {
    |                                 ^^^^ required by this bound in `View`

error[E0277]: `Rc<(dyn for<'a> Fn(&'a mut Cursive, usize, usize) + 'static)>` cannot be sent between threads safely
   --> /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cursive_table_view-0.14.0/src/lib.rs:995:21
    |
995 | impl<T, H> View for TableView<T, H>
    |                     ^^^^^^^^^^^^^^^ `Rc<(dyn for<'a> Fn(&'a mut Cursive, usize, usize) + 'static)>` cannot be sent between threads safely
    |
    = help: within `TableView<T, H>`, the trait `Send` is not implemented for `Rc<(dyn for<'a> Fn(&'a mut Cursive, usize, usize) + 'static)>`
note: required because it appears within the type `Option<Rc<(dyn for<'a> Fn(&'a mut Cursive, usize, usize) + 'static)>>`
   --> /usr/lib/rust/1.76.0/lib/rustlib/src/rust/library/core/src/option.rs:570:10
    |
570 | pub enum Option<T> {
    |          ^^^^^^
note: required because it appears within the type `TableView<T, H>`
   --> /home/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cursive_table_view-0.14.0/src/lib.rs:117:12
    |
117 | pub struct TableView<T, H> {
    |            ^^^^^^^^^
note: required by a bound in `cursive::View`
   --> /home/user/1tmp/cursive/cursive-core/src/view/view_trait.rs:34:33
    |
34  | pub trait View: Any + AnyView + Send {
    |                                 ^^^^ required by this bound in `View`

EDIT: Note however that I haven't tried to run the program(s) to see if it works correctly. It just builds ok, that's all I know :)