Don't ever reparse '/*!rules:re2c ... */' block.

Don't ever reparse '/*!rules:re2c ... */' block.

AST is immutable and independent of options: encoding expansion and other
option-sensitive transformations are delayed until AST is converted to
intermediate representation. Thus AST can be shared among different blocks
and conditions and reused with any set of options without modifications.

Changes in tests are trivial: warnings from the rules block are no longer
emitted (we don't compile AST to DFA for rules block anymore, since it
will be discarded anyway); skeleton tests no longer emit '.input' and
'.keys' files for rules block.