Reduce instruction size

[pjhades]

We will soon need to introduce character set like [a-zA-Z] and \w or even [\w\d\s] so there might be a new variant Inst::Char(CharKind::Charset(...)) which will definitely bloat the size of struct Inst.

The size of this struct should be kept as small as possible to reduce the overall memory consumption for the compiled program. So it may be helpful to have a CharKind::Charset(Box<HashSet<char>>) instead of CharKind::Charset(HashSet<char>).

Also, we can compile to bytecode so that each Inst variant takes 2 bytes. A trick could be played on the Split instructions. Since we're using i32 to represent an instruction address, the valid range is 0x00000000 - 0x7fffffff. So we can use the sign bit to record whether the next instruction is preferred. In this way we only need to record one Iaddr on a Split. Also 2 bytes is enough for holding a Box<HashSet<char>>.