This is necessary to fully support go1.21 and beyond, which now
provide some generic packages such as `cmp`, `maps` or `slices`
in the standard library.

The principle is to embed the generic symbols in source form (as
strings) so they can be instantiated as required during interpretation.

Extract() has been modified to skip the generic types, functions and
constraint interfaces which can't be represented as reflect.Values.

A new stdlib/generic package has been added to provide the corresponding
source files as embedded strings.

The `Use()` function has been changed to pre-parse generic symbols as
doing lazy parsing was causing cyclic dependencies issues at compiling.
This is something we may improve in the future.

A unit test using `cmp` has been added.

For now, there are still some issues with generic stdlib packages
inter-dependencies, for example `slices` importing `cmp`, or when
generic types or function signatures depends on pre-compiled types
in the same package, which we will support shortly.

In go1.21, the reflect rtype definition has been move to internal/abi.
We follow this change for maintainability, even if there is no layout
change (the go1.20 unsafe2 is compatible with go1.21).

We have isolated a few problematic tests which are failing sometimes
in go1.21, but work in go1.20, and also in go1.22. Those tests are
skipped if in go1.21. A preliminary investigation can not confirm that
something is wrong in yaegi, and the problem disappears with go1.22.