Split maybe_const_fn! into its own crate.

Changes

• The maybe_const_fn! macro has been moved to its own crate.
  For compatibility reasons, this macro is re-exported from the rustversion-detect crate (but still deprecated)
  • It is possible to disable this re-exporting and remove the dependency by turning off the default feature compat-maybe-const-fn.
• Improved crate-level documentation and README.

Removed

• Hide documentation for deprecated maybe_const_fn! macro.