Add prototype for unicode character parser (no API yet).

[renggli]

No description provided.

[renggli]

Regarding performance: it is mostly fastParseOn that suffers from the unicode character parser, not parseOn:

parseOn
-------
SingleCharacterParser              21.051μs [20.872μs; 21.230μs]
UnicodeCharacterParser (single)    29.786μs [28.004μs; 31.568μs]
                                   -41.586% [-50.249%; -32.923%]
UnicodeCharacterParser (surrogate) 39.743μs [39.516μs; 39.970μs]
                                   -88.881% [-90.784%; -86.979%]

fastParseOn
-----------
SingleCharacterParser              1.848μs [1.784μs; 1.913μs]
UnicodeCharacterParser (single)    8.512μs [8.327μs; 8.697μs]
                                   -362.589% [-375.351%; -349.828%]
UnicodeCharacterParser (surrogate) 8.729μs [8.388μs; 9.070μs]
                                   -375.337% [-398.632%; -352.042%]