rewrite prefixExpression parser rule using strategy from #112
use lookup table for postfix constructors
add a precedence table to be used when folding raw binary expr parses
extract operators as rules to DRY -NoImplicitObjectCall rules
rewrite binary expression parsing sans assignment and chained comparison
move chained comparison handling into reduceBinaryExpr
precedence table now maps operators (not operations) to precedences
define compound assign ops in prep for removing compound assign rules
revert: actually, we're not going to pull [compound] assign or seq in

The assignments are all particular about their LHS, so it's not
appropriate to treat them as any old binary operator. And since seqs are
below assigns in precedence order, they'll stay where they are as well.
Merge branch 'GH-112'
merge CS.ExistsAssignOp into CS.CompoundAssignOp