Clean up top-level modules

[mlms13]

• Decode is a bad name for the top-level module because it could easily conflict with other Decode modules (e.g. in a downstream project that depends on bs-decode)
• Decode.ParseError might not be a very useful alias in the top-level module, because parse errors are only useful inside the context of Decode.AsResult.OfParseError, which provides its own alias
• DecodeBase isn't aliased anywhere, so if you want to construct your own custom decoders, you have to use a second global module

Proposal:

• [x] Rename DecodeBase to Decode_Base
• [x] Alias Decode_Base inside the top-level Decode as Base
• [x] While we're renaming things, maybe Base should be Make
• [ ] Update some docs, probably

This will be a breaking change (but one that should be a super easy migration for most projects), so I'd like to get it in before 1.0.

[mlms13]

I've tentatively decided that Decode is an ok module name to claim, so prefixing with Bs everywhere maybe isn't needed. But there are still some other code cleanup suggestions here, so I won't close this ticket just yet.

[mlms13]

DecodeBase is now Decode.Base, and to construct a custom one, you use Decode_Base.Make, which is aliased as Decode.Make. The bsconfig.json has been updated to only expose Decode in order to avoid polluting the public namespace.