MemoryBuffer::free() conflicts.

[FrankStain]

https://github.com/cppformat/cppformat/blob/master/format.h#L462

Using MS Visual Studio project with turned on memory leaks monitoring its unable to compile format.h cuz free() now becomes a macro _free_dbg( p, _NORMAL_BLOCK ).

Suggest to rename this method into free_buffer().

Reproducing steps:

1- Modify format.h like that:
Was:

#ifndef FMT_FORMAT_H_
#define FMT_FORMAT_H_

#include <stdint.h>

Now:

#ifndef FMT_FORMAT_H_
#define FMT_FORMAT_H_

#define _CRTDBG_MAP_ALLOC
#include <crtdbg.h>

#include <stdint.h>

2- Try to compile format.cc using MS Visual Studio 2013+ (2015 used).
It will be failed with next messages:

format.h(465): warning C4003: not enough actual parameters for macro 'free'
format.h(475): warning C4003: not enough actual parameters for macro 'free'
format.h(504): warning C4003: not enough actual parameters for macro 'free'
format.h(465): error C2059: syntax error: ','

[vitaut]

Oh my, MSVC is a mess. Fixed in 77b3200, thanks!