Introduce format strings (for io:format etc.) and their types

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Types and primitives:

type format :: (o) -> o

format<$a, $b> : fun(format<$a>, $a) -> [$b]unit

Sample Usages:

f'key: ~s, value: ~p' : format<(string, $a)>

let show_mapping<$a, $b>(k : string, v : $a) : [$b]unit =
  format(f'key: ~s, value: ~p', (k, v))

[gfngfn]

Added at a3aa00b.