Moves Http method bitmask constants to Httplib

- Renames `HttpMethod` type to `HttpMethodBitmask`, `HttpMethods` to `HttpMethodBitmasks`
- Adds `HttpMethodNameToBitmask` and `HttpMethodBitmaskToName` Dicts for reverse lookup
- Specifies `STATUS_CODES` as a const
- Micro now actually uses `req.method` for route lookup instead of hard-coded `GET`