Differentiate on metadata

[pselle]

Differentiate on metadata, addresses #56

In prior behavior, this library would say that versions 1.0+abc and 1.0+def are equal, which can be an issue when checking which version should be used, such as in hashicorp/terraform#20814

An open question here is if this library means to say that 1.0+abc and 1.0+def are equivalent -- I am inclined to say not, but I am interested in prior decisions/data on this.

This PR repurposes comparePrereleases to check metadata (not just pre-release), and since that function does its own equality check, removes some of the pre-comparisons that were made (ex. checking if prerelease was "" before calling this compare). comparePrereleases and what it calls, comparePart, do the work of comparing strings for 1/0/-1.

Open to discussion on what user expected behavior is here!

[pselle]

According to the semver spec, https://semver.org/#spec-item-10

Build metadata SHOULD be ignored when determining version precedence. Thus two versions that differ only in the build metadata, have the same precedence. Examples: 1.0.0-alpha+001, 1.0.0+20130313144700, 1.0.0-beta+exp.sha.5114f85.

Thus, the existing behavior (as far as ordering goes) follows spec, so I'm closing this PR, but will look into this further.