Fold up a bunch of LLVM backend state into HLLVMParserCompileContext, and factor out some common IR emission

Implement IR generation for CHARSET_ACTION_SCAN, CHARSET_ACTION_ACCEPT and CHARSET_ACTION_COMPLEMENT

ch_llvm JITs successfully, but its call to h_read_bits crashes; the HInputStream seems to be invalidated

fixing LLVM function declarations; turns out repeat calls to LLVMPointerType(...) create shadow declarations