Run CI with gcc and clang on all branches

[ByteHamster]

• Run CI with gcc and clang on all branches
  • Fix compile errors to make CI pass. This includes merging fixed a weird bug #75, even though the change makes no sense. However, because fixed a weird bug #75 fixes a crash when compiling with -O3, it is the best option we currently have. It, does not have an impact on the performance.
• Treat warnings as errors when compiling PTHash itself. This does not effect using PTHash as a library in other projects. It only makes sure that future PRs to this repo don't introduce new warnings (that nobody would look into because the CI would still pass) that would then only surface in projects that use PTHash as a dependency.
• Fix undefined behavior caused by calling internal_memory_builder_single_phf<hasher_type, Bucketer>().swap(builder), which swaps a new builder (which has a bucketer with uninitialized values) with another builder. This causes uninitialized memory access and therefore undefined behavior. Initializing the bucketer members fixes this.

[ByteHamster]

@jermp This PR makes the dev branch compile with gcc and clang :) (plus CI to check this)

[jermp]

Thank you Hans-Peter for these additions. Very helpful. Can you please fix the initializations to the bucketers? We should use default constructors.

[ByteHamster]

Done :) Is this what you meant?

[jermp]

Perfect; thanks!

[jermp]

@ByteHamster and @stefanfred:
The classes skew_bucketer and uniform_bucketer are already correctly initialized since they have a default constructor. These initializations are therefore redundant. I would add default constructors to all the others that do not have it already.