Delete `extra_ops.py`, replace with specialized files with more informative names

[jessegrabowski]

Description

One might be indexing.py or index_tricks.py. That could hold anything related to generating indices or masks (including the new _delta and _iota functions introduced in #722.

There could also be a file called makers.py to hold ops like pt.linspace that make a useful tensor. pt.arange kind of straddles between an index and a tensor used in actual computation, so we could have a big debate about where it could go!

Other themes from stuff in extra ops:

• cumulative ops -- these should probably be in basic? Or a file for ops that operate on axes (cum, diff, unique)
• sorting related (sort, searchsorted, binsort) could be in a sort.py
• Several functions related to broadcasting -- broadcasting.py?
• Shape related - ravel, unravel, squeeze
• Fill diag/off-diag sort of falls under the umbrella of indexing if you think about it. But it probably belongs wherever Alloc is defined?
• Real grab-bag functions: CpuContiguous (is this used anywhere?), Bartlett (windowing function -- is this a refugee from the deleted signal.py?)

Needs some thought, obviously these are just some ideas to start a discussion.