Implement `va_list` and related functions

[ChAoSUnItY]

Currently in shecc, variadic functions is lack of flexibility, since variadic arguments cannot be passed from a variadic function to anthoer directly. This means we'll need a shecc-defined va_list and functions like vprintf in order to improve current error diagnostic mechanism in shecc.

However, due to current shecc's frontend, we may need to find a way to properly address issues of macro definitions such as va_start and va_end.