Add support for passing ABI version to `FFI.map_library_name`.

..until necessary native methods are added.

For consistency and to ease proting to JRuby and Truffleruby.

of commit e987ab50366a4b08617a20568eabdaa1fb761317

Freeze objects which are immutable from the start are now freezed.
This allows these objects to be used by Ractor without make_shareable.

This partially reverts commit 4fc6a8c5ec8a9a720330946af9d1103015c62942 in such a way, that functions are stored in a module variable @ffi_functions.
Enums are implemented per FFI::Library not per attached function.
To make them shareable they would have to be copied and freezed per function.
This would increase memory footprint for the sake of Ractor support.

IMHO it's better to mark the module explicit as finished by .freeze to allow its use in a Ractor.
This also enables querying the enum definitions from a Ractor.

Using a Hash instead of per-function variables allow to use foo! and bar? methods as wished in #971.

Fixes #971

Remove no longer used StructField.get and .put

It was introduced in commit 9d68c7f87ad7c869ee1ce3139269525af70e5df4 but moved to ruby code in commit 3f96bbb61b2f17ba9761a1d02745101a60cd26e1 .
These fields are a leftover from the commit.

Allow private release method for ManagedStruct and AutoPointer