[Parser] Parse functions and implicit types (#4972)

Implement function parsing, including parsing of locals and type uses. Also add
a new phase of parsing that iterates through type uses that do not have explicit
types to create any implicitly defined types and append them to the type index
space. This is important because the implicitly defined types may be referred to
by index elsewhere and because the legacy parser does not handle these
implicitly defined types correctly. Finally, maintain a map of implicit type use
locations to corresponding types for use in later phases of parsing.

Author: tlively