semver: ignore build metadata

[omasanori]

To provide strict total ordering for semver::Version, build metadata must be ignored for equality. Which way is preferred?

• Preserve build metadata in Version but equality checking ignores it.
  • Most of behaviors are preserved.
• Remove build metadata field from Version and the parser ignores it.
  • format!("{}", parse("1.0.0+build.1").unwrap()) == "1.0.0"

cc #12435

[erickt]

I we should keep the build data and exclude it from the equality and ordering check. That way we can convert a version back into a string.

[omasanori]

@erickt Sure.