Add mingw-std-threads project to external/ for full C++11 thread support, fix
an incorrect Win32 fix I did in PEGTL, work around mingw’s <io.h> declaring a
conflicting eof symbol, and fix a capitalisation error in log.cpp.

The containers no longer force the user to use the ptrdiff_t type for
size information. For convenience, size() now always returns an int,
and the size_s() method offers a way to handle arrays with more than
2 billion elements. Internally, we still use ptrdiff_t, though.

Since so much code had to be changed, I took the opportunity to get
rid of capitalised accessors for classes that are lowercase.

point parsing, add support for unary plus and minus.

MASK to SWIZZLE in the vector templates, rename matrix<> to mat<> for
consistency, implement transposition for all matrix sizes, make matrix
columns private and only accessible through operator[].