Create a single-threaded, "fast" mode for PASTA

[pgoodman]

Goals:

• Lots and lots of very tiny performance improvements

Possible approach:

• Disable std::mutex uses
• Change std::shared_ptr to raw pointers, where there could be a single separate thing to hold onto the lifetime of the AST

Needed changes to multiplier:

• Don't share the same PASTA filesystems across threads in the indexer