[mypyc] Add str.isdigit() primitive

[VaggelisD]

Similar issue as the str.isalnum() PR, for large enough strings the primitive introduces a perf regression but is generally faster on common cases:

All-digit strings (100M calls each)	Python (s)	mypyc (s)	Speedup
length 1 ('0')	2.089	0.656	3.2x
length 10 ('1234567890')	2.475	1.028	2.4x
length 100 ('5' * 100)	6.106	3.406	1.8x
length 1 (UCS-2: U+0660 ٠)	2.110	0.734	2.9x
length 10 (UCS-2: U+0660 * 10)	2.907	2.041	1.4x
length 100 (UCS-2: U+0660 * 100)	10.596	13.887	0.8x

Non-digit strings (100M calls each)	Python (s)	mypyc (s)	Speedup
length 1 ('a')	2.022	0.585	3.5x
length 100 ('a' * 100)	2.068	0.587	3.5x
length 100 ('0' * 99 + 'a')	7.304	3.474	2.1x

[VaggelisD]

There's a recurring pattern for these primitives, should we try to abstract their codegen?

Gave macros a shot for to hide the per-kind for loop, though we could go a step further and do the same for entire functions I guess.