PoC: Simplification & Speed Improvements

[arBmind]

This PR contains all my work for Virdigris so far.

• StaticStrings allows to store multiple strings in as on char-Array.
• StaticStrings extraction from variable amount of Literal & StaticString arguments is done in two fast phases without any recursion
• All registrations with W_-macros is stored using forward counters that avoid the binary tree
• The QMetaObject is generated entirely with pack expansions

For now this is a proof of concept with C++17. This makes a lot of code much simpler.

Results (timing on my machine):

• Visual Studio 2019: 19 sec (master 2:06 min - 6x faster)
• Visual Studio 2017: 20 sec (master 2:20 min - 7x faster)
• MinGW/gcc 7.3: 25 sec (master 1:13 min - 3x faster)
• Gcc 7 & 8 and Clang 7 & 8 run on Travis.

Next steps:

• [x] implement calls and casts without binary tree
• [x] remove binary tree
• [x] clean up recursive parameter generator

[arBmind]

See #58 for the new counter based registration.

[ogoffart]

Now you need to make it work with C++14 :-) I'd accept if there was some #ifdef #else for C++17 to get faster compilation with C++17.

Also new features can be C++17 only.

But i'd rather not mandate C++17 for old features.