New option "prefix" for enum/union constructors

[pictyeye]

The idea would be to write

enum dvi_opcode [prefix "OP_"] (8, UnknwonVal Opcode) =
...
| 138 -> nop
| 139 -> bop
| 140 -> eop
...

instead of

enum dvi_opcode (8, UnknwonVal OP_Opcode) =
...
| 138 -> OP_nop, "nop"
| 139 -> OP_bop, "bop"
| 140 -> OP_eop, "eop"
...