`str` returns unicode even when printing to LaTeX

[kpeeters]

The following makes LaTeX go belly-up:

ex:=\delta{x};
print(ex)

because print calls str to convert to printable, and (in the notebook) then feeds that into a verbatim environment, but str has used UTF-8 to represent the delta symbol, which LaTeX doesn't like.

[kpeeters]

This now works correctly in the microtex branch, which will shortly become the default.