Handle different size_t sizes correctly when setting up LLVM types, and include LLVM types for HBytes struct

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

Add --disable-llvm-backend/--enable-llvm-backend build-time options, and move llvm.c to src/backends/llvm/ subdir