Clarify the local_index space.

The design documentation does not appear to define the index space of 'locals' well.

This zero base index space should include the function parameters first and these are defined in the function section, and then an index for each local variable defined in the function body in the code section.

The names section also uses the local_index space, and I presume this space also includes the function parameters so function parameters can be named too. Importantly this means that the parameters for imported functions can be named in the names section, so imported functions can have a non-zero local_count value.

Closing, assume it will be sorted out in #750 or be subsumed by more general support for custom debug names.