Expand `SymbolTable`s to include all transactions in a `.prot` file

Right now, we create a new `SymbolTable` for each transaction (`fn` definition) in a `.prot` file. Ideally, we'd like to merge `SymbolTable`s for transactions within the same file into one big `SymbolTable`, although we'll need to handle issues w/ scoping:

Start by locating where a SymbolTable is created for each fn definition while processing a .prot file, then trace the existing scope handling. Determine how transactions can share one file-level table without leaking names across scopes; done means all transactions in a file use the merged table with correct scoping.

Written by the indexing model from the issue text.