`const`ify some of the methods

[Kyllingene]

I'm currently writing a voxel terrain generator (shush), and naturally I'm using perlin noise for that. Your crate has a very nice API, and thanks to its simplicity, converting most of the methods into const methods should be fairly straightforward.

I'd be glad to PR this myself, if you'd like.

[Auburn]

Sure, if you want to PR that would be great

[Keavon]

Please do submit a PR for this! I'll review it when it's ready, and we can publish a new release after the just-merged #138 and your PR are both landed.

[Kyllingene]

I'll do that this evening. One thing to note is that while this isn't a breaking change, reverting it will be; I'll take care to mention any possible pain points in my PR.

[Keavon]

Yep, as long as we're only adding const (which there shouldn't be any need to revert), this can be a patch release.

@Auburn I think the best way to handle versioning when faced with the many different languages is that you plan to keep the the x.y part of semver's x.y.z format in sync across all ports, but the .z part can freely get out of sync between ports. Maybe the readme can mention that in the part that lists each language.