Export a single function: parse_functions() which results a vector of
functions parsed from the source string.
Hide the parser and lexer modules. They are not useful to external
clients.
Replace the three tables instructions, extended_basic_blocks, and
extended_values with a single 'dfg' public member.
Clients using Function are changed to refer to func.layout and func.dfg
respectively.
The Layout also handles EBB layout, so new append_ebb calls are necessary.
- Rewrite callers to use the public data member 'layout'.
- Implement Debug for Function in terms of the write module to avoid deriving
it.
