[Parser] Parse blocks (#5393)

Parse both the folded and unfolded forms of blocks and structure the code to
make supporting additional block instructions like if-else and try-catch
relatively simple.

Parsing block types is extra fun because they may implicitly define new
signature heap types via a typeuse, but only if their types are not given by a
single result type. To figuring out whether a new type may be introduced in all
the relevant parsing stages, always track at least the arity of parsed results.

The parser parses block labels, but more work will be required to support branch
instructions that use them.

0 files changed, 0 insertions, 0 deletions