Add built-in lerp operator to Halide. Documentation in IROperator.h doc comment.

Includes correctness tests and tests for compilation error messages.

Also made two changes to allow writing a test cases that takes and
returns bool values. (Which are packed in an 8-bit byte.)
Merge branch 'lerp'
Changed bilateral grid to use the new lerp.
Changed the casts in the lerp code to catch vectorization of mixed
integer lerps.
Added a test for vectorized lerp.