Instruction formats are now identified by a signature that doesn't
include the ordering of value operands relative to immediate operands.
This means that the BinaryRev instruction format becomes redundant, so
delete it. The isub_imm instruction was the only one using that format.
Rename it to irsub_imm to make it clear what it does now that it is
printed as 'irsub_imm v2, 45'.
Add a Function::display() method which can include ISA-specific
information when printing the function.
If a test file has a unique ISA, use that in the `test cat`
implementation.
This makes it possible to refer to entities defined in the source file,
using the source names prefixed with $.
For example, $v20 refers to the value by that name in the sources, even
if it was renumbered to 'vx0' in the parsed file.
Create a new directory hierarchy under 'filetests' for all the tests
that are run by 'cton-util test'.
Convert the parser tests under 'tests/parser' to use 'test cat' and
filecheck directives.
