Define maximum length of namespace names and simple identifiers

Currently the prose specification defines no limit for namespace names, and the CSDL schema definition limits them to 512 characters.
Both the prose spec and the schema limit simple identifiers to 479 characters.

Apparently the C# compiler accepts only 511 characters for both variable names and namespace names, so 512 is too long for namespaces, and 479 is oddly short for a simple identifier.

JavaScript on the other hand has no limit for variable names.