crucible-llvm: Move translation from llvm-pretty ASTs into a separate library

[langston-barrett]

Crucible-LLVM now has several clients that do not make use of the translation from llvm-pretty ASTs into Crucible-LLVM ASTs (much of which happens in Lang.Crucible.LLVM.Translation.*):

• crucible-llvm-{cli,syntax} parse Crucible-LLVM CFGs using an S-expression syntax
• macaw-symbolic uses just the memory model
• crucible-wasm uses just the memory model

These clients would have faster build times if they could depend on a slimmer library that exports just the functionality they need. Furthermore, it would force us to think harder about our APIs (in a good way).