Add a Rust version check to cargo-pgx

[thomcc]

I had done part of this a while ago, and it came up recently, so I cleaned it off.

Fixes #774, although in the future I would like to relax this requirement and remove this check.

It does not detect mismatches between 1.x.0 and 1.x.1 (which should be fine), or between different betas/nightlies (which is probably strictly required, but would be personally pretty annoying for me).

It tends to ignore errors rather than failing, in case something in the output of one of the tools changes (unlikely, given how much of the ecosystem uses checks like this). There's also an escape-hatch environment variable called PGX_IGNORE_RUST_VERSIONS to disable the check.

I only check in the schema generation code, since that's the only code that needs to care about this.

[eeeebbbbrrrr]

Perhaps a little note in cargo-pgx/README.md about this general change is also necessary. When an issue gets reported about this (and it will), I'd like to be able to point that person to our docs that says, "see, this check is on purpose".

[eeeebbbbrrrr]

Thanks for the README words. Perfect!