hack unused-value warning on POP() to pass build

I'd like to build wren and play around it but encountered two problems, this PR makes me pass the build :-)

The first error is "src/wren_utils.h:50:22: error: unknown type name 'uint8_t'", which is fixed by adding the stdint.h header.

The second one is actually an unused-value warning but turns to error due to the -Werror CFLAGS in Makefile. It's caused by several calls to POP() without using its value. My workaroud is based on [GCC documentation](http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html):

> -Wunused-value
> ...To suppress this warning cast the unused expression to ‘void’. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as ‘x[i,j]’ causes a warning, while ‘x[(void)i,j]’ does not.