Replaced 'int' type with 'intptr_t' type in pointer arithmetic for compiling without -fpermisive + revisions by McNeight

[s-t-a-n]

Compiler yabs about losing precision:

~/Arduino/libraries/MemoryFree/src/MemoryFree.cpp:44:7: error: cast from 'void*' to 'int' loses precision [-fpermissive]

Using intptr_t type (as per C99 I believe) is recommended because it guarantees to hold the size of a pointer.

The Arduino IDE compiles with -fpermisive by default, but continuous integration frameworks like Arduino-CI and Cmake/Makefile type compilation does not.

Tested in Arduino-CI and on an Arduino Mega2560.

EDIT: I accidentally opened a pull request for this repo whilst I meant to open it for McNeights. Close it if you like. Pull it if you want.

[s-t-a-n]

oops wrong repo sorry :(

EDIT: I'll leave it here since it seems to contain usefull edits.