Decrease build time for parser

[01mf02]

A cold run of cargo build --release takes about three minutes.
A lot of this time is spent building jaq-parse.
Luckily, cargo build (without --release) is much faster, but still, it would be nice to have shorter build times in release mode.
I suspect that the types in jaq-parse getting too large is the culprit. It might help to insert boxed() calls here and there to remedy the problem, but the last time I tried that, I got lifetime errors.