Parser: duplicate TypeStatementImpl objects

Analysis of a memory snapshot with vendor models shows we end up with a SchemaContext which weighs about 240MB. Further analysis of duplicate objects shows we have ~12000 instances of TypeStatementImpl wasting ~2MB, which refer to simple yang types:
argument&rawArgument == "string", "uint32", etc.
source == DECLARATION
definition == TYPE
substatemebts == ImmutableList.of()

Pre-allocate instances for all YANG basic types (e.g. not decimal64 and similar) and make sure to squash these duplicates to singleton instances.