Add instruction names

[ThinkOpenly]

It would be really nice to be able to extract the names of the respective instructions from their definitions. One suggestion is to use "attributes":

$[name jump and link register]
union clause ast = RISCV_JALR : (bits(12), regidx, regidx)

[ThinkOpenly]

Commit 65fca04 includes some additional names, and a mechanism for adding names for instructions whose mnemonics are within mappings:

mapping itype_mnemonic : iop <-> string = {
  $[name add immediate]
    RISCV_ADDI  <-> "addi",
  $[name set less than immediate]
    RISCV_SLTI  <-> "slti",
[...]