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Removed a special case in the parser for FuncRefV constants, which
required every SSA variable to have a known type. This required
BundleParser to be rearranged to parse everything else before function
bodies, making functions available to functions defined above them
(something the spec requires, so that's progress!)

Also added support for COMMINST in the parser.

The current version of the Mu specification does not use $-variables to
pass return values from terminsts to the normal destination of an EXC
clause, instead allowing terminsts to have normal SSA return values
that may be referenced in the EXC clause. This commit updates the
formalism to match.

Several other refactorings are included, such as making exec_terminst
only return modified stacks, using a simpler representation of common
instructions, and defining prefix/infix operators for SSA variables and
registers in HOL.

The NEWTHREAD instruction isn't parsed yet, but its associated memory message is
now supported, and the scheduler has been modified to support multiple threads.

thread_states are now generated on-demand when they are first executed, provided
there is an associated DoNewThread memory action and its stack ID can be
resolved. This required the 'tid' field of 'Message' to be changed to a
'thread_id option', as there must be an initial DoNewThread action that is not
generated by any thread ID.

The THROW instruction and EXC resume_values have also been updated to have only
one argument, to match the spec.