Make Sass_Values const if possible in API function calls

This is in reference to https://github.com/sass/libsass/issues/689

I'm not 100% sure if this really is the right way to "solve" this issue, but it should at least improve the situation slightly.
Compiler will now issue a warning if someone tries to return the same value in custom functions ... if warnings are enabled.
Any opinions on this? I'm just not sure what the best way is to tell implementors where a copy is needed and where it is not!?