Move backend relations to compiler/backend/languages/relations

This issue is about moving all syntactic relations to compiler/backend/languages/relations.

In that directory:
- each file should only have one syntactic relation it "exports"
- each file has 3 sections:
1. the definition of relation of interest from outside
2. the proofs (for internal use to this file only, can be local)
3. the from outside interesting thm
- naming conventions:
- file name should start with language name
- exported exp_rel should have descriptive short name (not exp_rel)

Start by inspecting the existing compiler backend language files and inventorying the syntactic relations that belong in compiler/backend/languages/relations. Organize the target directory so each file has one exported relation, the three requested sections, language-prefixed filenames, and descriptive exported names. Done means all syntactic relations follow these conventions.

Written by the indexing model from the issue text.