Cleanup: Animation, simplified fcurve_eval_between_keyframes() code

The function now immediately returns the evaluated value, rather than
setting a variable's value and returning that at the end of the
function. As a result, `else`-clauses and `break` statements could be
removed, simplifying and unindenting the code.

Declarations of some variables that are only used in a specific part of
the function have been moved there to clarify and limit their scope.

No functional changes.

Animation: added unittests for FCurve evaluation

I want to clean up some of the FCurve evaluation functions by breaking
them up into smaller parts and simplifying some code paths. Before I do
that, however, I want to add some tests to ensure I don't mess up too
much.

Cleanup: Animation, split fcurve_eval_keyframes() into separate functions

There are three main parts of the `fcurve_eval_keyframes()` function:

- before or on the first keyframe
- after or on the last keyframe
- between those points

These are now separated into separate functions. Apart from moving the
code, there are minimal code/cleanup changes. More cleanups to follow.

No functional changes.

Cleanup: Animation, removed impossible-to-reach code

While fixing T39207 @aligorith added a threshold to the binary search
function that's used to find the closest keyframe. Since that threshold
is larger than the threshold used in the now-removed code, the condition
in that code will evaluate to true anyway, and thus it can be removed.