`evalbuf` should be replaced.

[dancrossnyc]

evalbuf is a global character buffer that is used in conjunction with custom output routines that, if passed a nil FILE output stream pointer, will instead concatenate to the buffer. It is then treated as a source for execution later.

Instead of using FILE * for output everywhere, we should move to std::ostream; we can then pass a std::ostringstream where we'd pass nullptr and implicitly copy into evalbuf. This would make output reentrant, and eliminate the need for special-case output functions (wfputs, etc).