Remove dependency on mruby-*-ext mrbgems

[lopopolo]

These implementations of Ruby core methods have been mostly buggy and not spec-compliant.

Use them as a starting point to implement them natively in Rust/evaled Ruby with the goal of passing ruby/spec.

• [x] mruby-compar-ext
• [x] mruby-enum-ext
• [x] mruby-numeric-ext
• [x] mruby-array-ext
• [x] mruby-hash-ext
• [x] mruby-range-ext
• [ ] mruby-proc-ext
• [x] mruby-symbol-ext
• [x] mruby-object-ext
• [x] mruby-kernel-ext
• [ ] mruby-class-ext

[lopopolo]

This change will allow eliminating dependency on mrbc which means:

• we can null out the host build for mruby-sys.
• no more dependency on gcc.

[tkbky]

I'm working on for mruby-enum-ext

[lopopolo]

Sounds good thanks @tkbky

[tkbky]

picking up mruby-numeric-ext

[lopopolo]

Thank you @tkbky

[lopopolo]

Updated the checklist with the great work @tkbky has been doing. 💎 💪

[lopopolo]

@tkbky has been making awesome progress on this ticket 🎉

[lopopolo]

We can't yet remove mruby-proc-ext from mruby-sys/build.rs yet because mruby-method depends on it.

Originally posted by @tkbky in https://github.com/artichoke/artichoke/pull/217#issuecomment-526615816