Proposal: procedural comptime type manipulation

[distractedlambda]

I'm rather new to Zig, so apologies if this is something that's already in the works or has already been discussed before.

I was surprised and happy to see the @field() builtin function, but it seems like only the start of what could be a much larger set of features. In particular:

• The ability to enumerate, inspect, and look up all public members of a type, not just fields.
• The ability to add and remove members, fields, enum cases, etc. from types. Since types are immutable, this would probably take the form of builtins that produce new anonymous types from existing ones.

There are a number of use cases I can think of:

• Automatically parsing and generating type definitions for schemas for external markup languages at comptime.
• Postprocessing the struct from an @cImport and generating a new "glue" interface. For instance, a comptime OpenGL extension loader generator that learns about available procedures by browsing the Zig definitions generated from the headers.
• Any embedded DSL where you need to generate types with computed member names.

[hryx]

Related: #383

[komuw]

probably also related; #2907

[andrewrk]

I believe this is a duplicate of #383.