Move most Interval method bodies to the .cpp file

[steven-johnson]

Saves ~12k from libHalide.so on x86-64-linux; timing on local_laplacian.generator doesn't show any performance impact.

[abadams]

I'm surprised by this one. Most of these methods are incredibly cheap, e.g. two pointer comparisons. I had assumed they were being inlined. Can we hold off on merging this one until I can investigate?

[abadams]

Slows down allocation bounds inference on local laplacian by 5% or so. To claw it back I just had to inline both constructors. That in fact gave a minor speed boost (1%) over master for that stage.

Turns out operator== exists because the built-in autoscheduler wants to test to map<string, Interval> for equality in order to cache computations. I'll add a note.

[steven-johnson]

No rush.

[steven-johnson]

Huh, clearly I didn't time carefully enough, my local-laplacian run seemed to be within typical noise variation.

[abadams]

I just put tic/toc around allocation bounds inference, which is the stage that uses intervals the most. The 5%/1% refers to that stage alone. It is indeed in the noise for overall compilation.