Cache parsed definitions from standard library

[01mf02]

This PR moves the parsing of the standard library from run-time to compile-time.
Parsing the standard library 1024 times takes 116ms before and 12ms after this change.

I'm not sure yet whether to merge this.

Advantages:

• startup time decreases a bit, but not too much; it is still about 9% above gojq (1.04s for jaq vs. 0.95s for gojq-0.12.16 when running {jq} -n empty 2048 times in a row)
• when printing a parsed term with dbg!, the output is now valid Rust code and can be copy-pasted

Disadvantages:

• binary size increases from 2.0MB to 2.2MB
• code size increases by about 100 lines
• build.rs is required in jaq-{std,json}
• symlink used for jaq-json/build.rs, this might not be portable

If you have any comments about this, then please feel free to add them!

[01mf02]

I have decided not to merge this. The disadvantages far outweigh the advantages.