Let primary APIs have the better module name (CodableStructuredHeaders -> StructuredHeaders)

[jshier]

As mentioned in the announcement thread, if users are intended to start with the higher level module, CodableStructuredHeaders, shouldn't it get the better module name? This increases discoverability (users don't need to know about the other module) and allows users to start at the high level rather than low. The current StructuredHeaders could be renamed to LowLevelStructuredHeaders or anything else that indicates it's lower level than the other module.