std::ascii:  Fix is_digit() and is_control()

is_digit() incorrectly returned false for '0'.
is_control() incorrectly returned true for ' ' (space).
std::ascii:  Add tests for is_digit() and is_control()
auto merge of #10685 : ebiggers/rust/ascii_fixes, r=alexcrichton

is_digit() incorrectly returned false for '0'.
is_control() incorrectly returned true for ' ' (space).