Check that elements produced by BuildStream are bit-for-bit reproducible.

We can do the check manually by doing two builds and checkouts, deleting the cache key in between. Provide a means to automate this.

Additionally, we should be able to use the reprotest (https://anonscm.debian.org/git/reproducible/reprotest.git) utility to vary build conditions for BuildStream as much as possible to find non-reproducible behaviour both in BuildStream and in elements.