Add a count-only mode to the Writer (advance pos without storing) and factor the
section walk into emit_module, so encode and the new encoded_size(m) share one
path. encoded_size returns the exact byte count encode will produce -- the SPIR-V
analog of an ISA's encode_max_code_size, here exact rather than an upper bound.
The printer now sizes its scratch buffer with it instead of retry-on-overflow.
Verified: encoded_size == encode byte_count; suite 10 passed.
d39cf98931