checks for clashes with other function overloads in fmt with templated parameters on the first or second position in the parameter list

std::back_insert_iterators model the OutputIterator concept but differ considerably in their traits and behavior. In particular the former made compilation to fail when format_to_n is given a back_inserter as first argument. The emulation of an OutputIterator is not perfect due to the behavioural differences of back_insert_iterators (e.g. assignment always implies increment) but good enough to be used within fmt's machinery.

Adapt any string-like type to be used by {fmt} just like the standard string types already supported. The adaption is totally non-intrusive.
Signed-off-by: Daniela Engert <dani@ngrt.de>

there is chance that the distro install python as python3, in that case
the shebang of
```
```
in build.py won't work.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>

this allows us to use sphinx-build-3, as user might already in a python3
world, and distros, like fedora 28, tend to add a "-3" suffix to sphinx-build.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>