Remove inappropriate exec. permission
.travis.yml: Switch to latest Agda version

Somehow, Agda does not install any more on Travis:
https://travis-ci.org/inc-lc/ilc-agda/builds/32258151
.travis.yml: Also install up-to-date happy

See
https://travis-ci.org/inc-lc/ilc-agda/builds/32259400
.travis.yml: Also install up-to-date alex

This exhausts the build-tools from Agda.cabal:

https://github.com/agda/agda/blob/2.4.0.2/Agda.cabal

Hopefully this will be fully sufficient, finally.
Add build status image to README.md

I noticed after looking for the "build: failing" image on the README.
(The build failure is stupid and is being fixed).
Draft conversion to CBPV types
Compile fix
Complete term conversion to CBPV (with hack for constants)
Prototype proof that static caching is type-correct

See comments for extra info.