Fix memory leaks, datatype, logging facility, documentation, ...

[lcsmuller]

What?

1. Use greatest.h for writing unit tests.
2. Convert bitmask enumerators to macros and replace enum type with u64bitmask.
3. Client logging can now be configured without a config.json file.

Why?

1. Unit tests will greatly benefit spotting regressions before its too late. In the future, a Discord mockup server such as Fosscord or Litecord can be used to make testing more flexible (eg. testing a scenario that requires scaleability).
2. Per the standard, enums are only guaranteed to be as large as a int - going well over some of Discord's bitwise values (such as Permissions)
3. See Cant set default prefix at config.json #25

How?

1. Rewrite rest.c test with greatest.h tools.
2. Rewrite enumerators as macros and introduce a new FIELD_BITMASK() gencodecs macro.
3. Concord's logging facility is modular, meaning there is a root logging module that can have N sub-modules linked to it (each module responsible for logging something unique). Client logging couldn't be configured manually because each module had its own copies of configuration values, instead of those being shared among them - this is now fixed.