Fix utility library

[Haibib]

PR Description

• Adds fix utils: a library for fix procedures to import. It includes utility functions and operations that are resolved to externrefs.
• Adds fix-postprocessor: adds WebAssembly memories to the given WebAssembly binary by parsing the wasm_num_memories custom section.
• Adds helper functions to safely get the contents of blobs and tree handles and to create handles from blob and tree contents by naively allocating new tables and memories.
• Makes the fix parser into a fix procedure that relies on the utility library to interact with the fix shell
• Updates the grammar used for the fix parser to reduce line noise.
• Adds selection thunk handling, primitive handling, and a basic $identity primitive to the fix parser