compute lookahead functions automatically

[bd82]

• this should be optional. can still explicitly define the lookahead function
• should only be computed once.
• won't work if the env disallows new Function()/eval should have a fallback mode that will still
  work in those kinds of envs.

[bd82]

Not using any eval/New Function so this should work everywhere.
first subitem: for OPTION DSL Rule:
#13

[bd82]

#16

[bd82]

#20