delete now obsolete _Substitution.pm6
delete now obsolete _FreeVars.pm6
fix tests for FreeVars
refactor FreeVars tests, to accomodate for lambda-style fns re free vars; remove tests on P6 Terms
move stuff from _EtaReduction.pm6 to LambdaModel.pm6
move stuff from _BetaReduction.pm6 to LambdaModel.pm6
use TermADT in Grammar, remove Tree.pm6, move VarT.get functionality to VarT ctor (impure!)