Fails to compile on a modern Linux system, fixed via PR

[paulie-g]

Various identifiers from Avian's vm::OpCode enum - fadd, fmul, fsub, fdiv - clash with fadd, fmul, fsub, fdiv declared elsewhere when compiling on a modern Linux system. This results in gcc complaining about ambiguity (existentially understandable, but not helpful in this case).

Minimum viable patch submitted via PR #568