Semver parsing / comparison

[ethanfrey]

Useful feature for migration support.

Pretty much every contract uses cw2 to store the version as defined by cargo:

const CONTRACT_VERSION: &str = env!("CARGO_PKG_VERSION");

This is a string however, and not the easiest to use in migrate. to say, "allow migration where version.major == 0 && [8..-=10].contains(version.minor)".

As a first step, is a utility to parse a valid semver string into:

pub Semver struct {
  pub major: u32;
  pub minor: u32;
  pub patch: u32;
  // if you think this needs even more structure here, feel free to add
  pub tag: Option<String>;
}

Also please add some basic constructors and implement Ord, so we can do something like:

if version < Semver::new(0, 8, 0) {
  return Err("too low");
} else if version >= Semver::new(0, 10, 1) {
  return Err("too high");
} else {
  // do migration
}

The types above are meant to give inspiration and you can change them if you have a better idea that fulfills the same purpose.

[ethanfrey]

Can be closed by using semver crate in #511