Benchmarking: We should benchmark NOW so that we know the broad strokes performance

# Why?

@arthurpaulino reports ominous performance shortcomings of megaparsec stack.

@cognivore has seen stack overflows while transpiling this December.

All in all, it hints that at some stages of running rust on lurk, we'll hit some bottlenecks.

We can actually benchmark NOW to see if stuff will work somewhat.

Here's what we need to do to achieve this.

# What?

- [ ] Mock everything that's not yet implemented in the list.
- [ ] Run e2e. Check that:
- [ ] Megaparsec parses WAST into Expr
- [ ] Yatima transpiles Megaparsec-enabled code into Lurk [optional]
- [ ] *and/or* Yatima transpiles Expr into Lurk (without transpiling Megaparsec) [required]
- [ ] Lurk runs transpiled code without choking.

# Who?

I can do this, but I need mocks to be pushed to some branch.

Start by locating the existing end-to-end path for Megaparsec, WAST, Expr, Yatima, and Lurk; no files or tests are named in the issue. Review which unimplemented components need mocks, then run the e2e flow. Done means the required Expr-to-Lurk transpilation path runs without choking, with the optional Megaparsec-enabled path checked if possible.

Written by the indexing model from the issue text.