add Collection interface

[alex-s168]

I broke a completely unrelated thing in the compiler wtih this:

./lm1 --c -o lm.c SRC/index-index.lm
Context::lookup alias-index : List<Tuple<String,String,Type>>+LMStruct+LMStruct+Collection<Tuple<String,String,Type>>+GlobalVariable+Sized
Candidates:
Context::lookup Could Not Find Symbol at In File SRC/alias.lsts Line 5 Column 4

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a new feature, I have added thorough tests.
• I agree to release these changes under the terms of the permissive MIT license (1).

1. https://github.com/andrew-johnson-4/lambda-mountain/blob/main/LICENSE

[andrew-johnson-4]

Things that need to happen before interfaces "just work"

• turn Collection<x> into Collection<x>+y in function parameters
• apparently this: erase Collection<x> during normalization (interfaces are denormal)