Shrink `Jim_Scope` size from 12 to 1 byte

[HalidOdat]

Hi @rexim :wave:

I came across this library while I was watching your youtube video on implementing GC in C (awesome video btw). And thought I'd check it out :)

While checking the code I found that we keep the scopes as three ints one for kind, one for tail and one for key. The size of this is 12 bytes. But we can store it as bit field. And keep 6 bits for the kind (enough to store 64 different kinds, we only have 2 now), 1 bit for the tail and another 1 bit for key (since key and tail can only be 1 or 0 we only need 1 bit). With this we save 11 bytes per scope. And since we have 128 by default, that's 1408 less bytes for the Jim struct.