Increase test image size for interpolate
halide: switch default compiler to c++ (from clang)
halide: track "program version" as hash of source

Use simple md5 of program.cpp as program_version in tuner database
halide: tuner improvements

- always keep failed schedule source (like debug_schedule)
- use unique tempfile for binfile - allows multiple tuner instances to run simultaneously on the same machine
- catch SIGALRM exit code -14 as another "program timeout" case