Restructured codebase

[RPG-Alex]

Previously the codebase was largle relegated to large module files, e.g. abs_diff_eq.rs, I have broken these up into modules like:

abs_diff_eq/
├── collections.rs
├── core_impls.rs
├── external.rs
├── mod.rs
├── primitives.rs
├── trait_def.rs
└── tuples.rs

There hasn't been any other major changes, it was mostly moving the code into the new home. This PR only covers approx and not the derive implementation. Will do derive next if this is acceptable.