Implement `ToStr`s for integer types in Fir

[osa1]

Currently we implement int-to-string convos in the backends:

fir/Fir/Num.fir

Lines 12 to 13 in d0047fc

	impl ToStr[I64]:
	prim toStr(self: I64) Str

That was easy back when I first added these, but now that we have two backends, and Fir compiles to quite fast C, it makes sense to implement them in Fir.