fix alias for Struct accessors

Correct a small error in parse.y

Correct a small error in parse.y, which causes the reading of unassigned memory (triggers an error when address sanitizer is active)

Fixes #3825

Bug/paren arg

`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