QV4::Value: Research different tagging scheme

Idea: Values store only pointers to managed objects, which have 16byte alignment => we have 3 bits for tagging.

x: any value

xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxx0

pointer (can be stored as-is)

xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01

int62?, only support int32 initially by storing it in the first half

xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxx011

double, probably has to be a pointer to a heap managed double.

xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxx111

we can use all those values for various tags (undefined, empty, nan, +/-inf, Null, true, false). Exact scheme to be decided.