Disallow invalid namespaces

The name portion of the fullname of named types, record field names, and enum symbols must:

    start with [A-Za-z_]
    subsequently contain only [A-Za-z0-9_]

A namespace is a dot-separated sequence of such names. The empty string may also be used as a namespace to indicate the null namespace. Equality of names (including field names and enum symbols) as well as fullnames is case-sensitive.

The null namespace may not be used in a dot-separated sequence of names. So the grammar for a namespace is:

  <empty> | <name>[(<dot><name>)*]