Specialize is_scm, from_scm, and to_scm for the 8 fundamental integer
types: {signed, unsigned} ⨯ {short, int, long, long long}.
Remove the specializations for size_t because it is a typedef.
Because the is_scm functions perform range checks, state the range
when one fails in LY_ASSERT_TYPE calls. This avoids insults like
Wrong type argument in position 1 (expecting integer): 9999999999Add a predicate for ly:wide-char->utf-8 and remove some C-style casts.