Update AVR GCC Toolchain to v3.6.x about fast-arduino-lib

[jfpoilpret]

This toolchain update is not going to be easy. As it turns out currently, it generates a load of "error: reinterpret_cast from integer to pointer" which are due to FastArduino way of handling REGISTERS as template parameters.

Older g++ versions would allow that (in fact it seems that was a mistake in these versions) but newer ones prevent it as per the standard.

So the issue is in FastArduino itself. I need to find a simple way to make it compliant to the standard, but for that I will first need to create a simple example showing the issue (outside FastArduino that just generates too many issues now).

I guess it will take more time as expected. Maybe release 1.1 shall be postponed or the issue assigned to a later milestone. Not sure yet what I will do. I will comment later upon my decision.

[jfpoilpret]

Support for newr avr-gcc (6.3.0) has been committed to master (ffd9371).

This was successfully tested with the following Fedora AVR packages:

• avr-binutils.x86_64 1:2.27-2.fc26
• avr-gcc.x86_64 1:6.3.0-1.fc26
• avr-gcc-c++.x86_64 1:6.3.0-1.fc26
• avr-libc.noarch 2.0.0-3.fc26

[jfpoilpret]

Next step in this issue is to try and improve codebase through new C++14 features like:

• namespace X::Y::Z {... syntax
• support for complex constexpr functions (several FastArduino constexpr functions, in order to compile with C++11, had to be split in several recursive constexpr sub-functions, which makes code harder to understand and maintain

The following points must be checked though:

• support of C++14 by doxygen
• support of C++14 by clang-format (FastArduino currently uses clang 4.0, which is pretty old)

[jfpoilpret]

Latest FastArduino commits are using AVR-GCC 7.2 (from Fedora distribution packages) in C++14 mode.