Split 'src/codegen/code.cc' into parts.

Split 'src/codegen/code.cc' into parts.

First, DFA is built ('re2c::DFA::DFA'), then it must be prepared
for code generation: some states must be split, backtracking points
must be marked, etc. ('re2c::DFA::prepare'), then finally code
can be generated ('re2c::DFA::genCode').

I haven't yet fully decided whether second stage (preparing) is
closer to DFA construction in general (and thus should be moved to
'src/dfa') or to code generation (and should be moved to 'src/codegen').
Since it deals a lot with bitmaps, second variant will suffice for
now. Perhaps later on I'll split preparation into general and
codegen-related parts.