Refactor code

[cgahr]

I'm currently going through the code to understand what's going on.

For reference, the current dependency graph in the project looks as follows (assume #84 gets merged; created using pydeps):

To be completely honest, I am very much confused by what does what. So I propose the following: refactor the code.

Step 1

As a first step, I'd like to remove ssort._utils and move its functions to different locations:
Right now there are the following functions defined in _utils.py:

cached_method
detect_encoding
detect_newline
escape_path
normalize_newline
single_dispatch
sort_key_from_iter

Each of these functions is only used in very few places:

cached_method:
    _statements.py
detect_encoding, detect_newline, normalize_newline:
    _main.py
    _ssort.py
escape_path:
    _main.py
single_dispatch:
    _ast.py
    _method_requirements.py
    _bindings.py
    _requirements.py
sort_key_from_iter:
    _graph.py

I propse to move these functions to the following files:

cached_method -> _statements.py
detect_encoding -> _files.py
detect_newline -> _files.py
escape_path -> _files.py
normalize_newline -> _files.py
single_dispatch -> _single_dispatch.py
sort_key_from_iter -> _graphs.py

The resulting graph looks as follows:

#91 implements these changes.

What do you think?