Add arbitrary precision support for serde_json deserialization

[tenuous-guidance]

With the 0.4 changes, parsing decimals from JSON results in artefacts from those numbers going via f64. serde-json has support for abitrary precision parsing, which would solve this without reverting to 0.3 behaviour.

This is something I might be able to work on, but I wanted to check if it would be considered first. I think the way rust-decimal handled it is fairly good, and so would probably copy that behaviour if this was considered reasonable.

This is somewhat a duplicate of #113 but wanted to raise it separately as it's more focussed on this particular approach to fixing it.

[akubera]

Yeah, that sounds good to me. I'm not familiar with implementing serde transformations. Is it essentially a function mapping &str -> BigDecimal?

I've been splitting code into "impl" modules instead of putting everything in lib.rs. You can make impl_serde.rs and put code and tests in there. Branch off of trunk.

I think we had a limit of serde_json = "<1.0.101" due to incompatibilities with minimum-supported-rust-version 1.43. If that's an issue we'll merge this into an 0.5.0 branch. I'm trying not to stray too far from the num_ crates' MSRV of 1.31.