Interface-to-interface type assertion

[quasilyte]

This feature require:

1. Each type full method table storage available at run-time due to
   method set matching.
2. Global method matching cache.

It is implementable, but additional global state seems obnoxious.
Most (99.9%) of Emacs code will not ever reference that metadata.

This issue state may change when reflect package will be developed.

[quasilyte]

In the current implementation, only precomputed method tables are used.