Some instructions are grouped in a single `mapping clause assembly`
where the respective mnemonics are embedded within a separate `mapping`.

For instructions which are _not_ grouped, the name of the instruction
can be added as an attribute to any of the instruction-specific constructs.

For grouped instructions, the attribute needs to be added to a construct
at the granularity of the specific instruction. Here, we attach the
attribute within the individual element of the `mapping` that includes
the actual instruction mnemonic.

This approach is still insufficient for mnemonics that are constructed:
```
mapping clause assembly = VLSEGTYPE(nf, vm, rs1, width, vd)
  <-> "vl" ^ nfields_string(nf) ^ "e" ^ vlewidth_bitsnumberstr(width) ^ ".v" [...]
```
...so more thought is needed here.

Also, I'm settling on a convention of using lower case (not Leading Caps),
at least for now. This matches how the instruction names are written in
the ISA specification, at least for those instructions for which the name
is actually provided. :-/