`RSTR_FSHARED` use frozen strings as shared body instead of
`struct mrb_shared_string`. This reduces allocation from
literal strings.

Fix MSVC 14.0 warnings

fix: src\string.c(2219): warning C4244: 'function': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\string.c(1924): warning C4244: '=': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\string.c(1130): warning C4244: '=': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\string.c(497): warning C4244: '=': conversion from 'mrb_int' to 'long', possible loss of data

fix: src\range.c(136): warning C4244: '=': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\proc.c(96): warning C4244: 'function': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\print.c(22): warning C4244: 'initializing': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\numeric.c(1215): warning C4244: 'function': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\numeric.c(954): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)

fix: src\numeric.c(897): warning C4244: 'function': conversion from 'mrb_int' to 'mrb_float', possible loss of data

fix: src\kernel.c(874): warning C4244: 'function': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\kernel.c(861): warning C4244: '=': conversion from 'mrb_int' to 'int', possible loss of data

fix: src\hash.c(139): warning C4244: 'function': conversion from 'mrb_int' to 'khint_t', possible loss of data

fix: src\hash.c(40): warning C4244: '=': conversion from 'mrb_int' to 'khint_t', possible loss of data

fix: src\hash.c(27): warning C4244: '=': conversion from 'mrb_int' to 'khint_t', possible loss of data

fix: src\dump.c(710): warning C4244: 'function': conversion from 'mrb_int' to 'uint16_t', possible loss of data