compilation fix for no-std

[michalkucharczyk]

This PR fixes following compilation problem for no-std environments:

error[E0412]: cannot find type `Vec` in this scope
   --> /home/miszka/parity/10-genesis-config/rust-bip39-release/src/lib.rs:110:26
    |
110 |     pub fn to_vec(&self) -> Vec<Language> {
    |                             ^^^ not found in this scope

error[E0599]: no method named `to_string` found for struct `Decompositions` in the current scope
    --> /home/miszka/parity/10-genesis-config/rust-bip39-release/src/lib.rs:201:42
     |
201  |             *cow = Cow::Owned(cow.as_ref().nfkd().to_string());
     |                                                   ^^^^^^^^^ method not found in `Decompositions<Chars<'_>>`

The problem can be reproduced with the following steps:

cargo new rust-bip39-dep-problem2
pushd rust-bip39-dep-problem2
cargo add bip39 --git "https://github.com/rust-bitcoin/rust-bip39"  --rev b100bf3 --no-default-features --features alloc
cargo build

[michalkucharczyk]

cc: @tcharding

Would be nice to add this check to CI somehow. Do you have any idea how to do it?
Maybe we could create a cargo new somewhere in tmp directory?

[tcharding]

ACK 8653fb2