Improve JSON handling.

 - The ~= operator is now enabled
 - Using opIndex to create a new field has received a workaround for a DMD bug
 - writeJsonString and writePrettyJsonString have been merged + unittests added
Fix __LINE__ static assert
Split `restutil` module into smaller parts.

Move those smaller modules into freshly created `vibe.utils.meta` package.

vibe.d code style was enforced in process, documented unittest feature used, few small bugs fixed.
fix excessive compile times, unneeded initialization

- Array initializer trigger O(N^2) behavior in dmd.
- Buffers don't need initialization.
Merge pull request #340 from Dicebot/meta-package

Split / move `restutil` module
Merge pull request #341 from dawgfoto/fasterCompilation

fix excessive compile times, unneeded initialization