Move backend relations to compiler/backend/languages/relations

[myreen]

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)

[hrutvik]

Also, pure_letrec_{seq,spec}Theory/pure_inlineTheory/pure_demandTheory should really be moved from meta-theory to join the other compiler relations