compiler: bulk rename stdgum to libgum in all places

bilby: add typeproof build option

bilby: makefile targets to check generated proofs

[skip ci][skip lemma]

bilby: string literals only in debug mode

The formalisation doesn't really support string literals, and C parser
doesn't support them properly, so even if we wanted to support them, we
would have to cheat.

As a result, all the debugging calls with string literals in Bilby were
causing issues with the proofs. I've wrapped them all in conditional
macros.

You can turn them on using `make DEBUG=1`.

I've also moved the definition of `cogent_debug_bool` to C, because the
string literals there were causing issues.

quickcheck-readpage: trying shallow embdg

[skip lemma]

bilby: fix Makefile.

We don't need to pass all .ac files to the command line, as they've
been included by the top-level .ac file

compiler: work on Haskell FFI generation

Unlikely to be bug-free. Need to test.
[skip lemma][skip ci]

compiler+fs: fix naming of generated files

[skip lemma]

impl: Use gcc headers from the correct version.

Get `cogent` path from build-env.sh

On machines where the `cogent` binary isn't in the standard path,
running `cogent --stdgum-dir` will fail. Inherit from `build-env.sh`
instead.

regression: lemma and travis cover most tests (minus huge proofs)

* add some isabelle related tests back to lemma-regression
* update libgum typecheck test
* fix isabelle tests
* fix bilby build (fail due to kernel compatibility)
* other misc fixes to test script

[impl] Reorganise impl directory.

We currently have all our filesystem implementation in `impl` directory. This
patch moves all the implementations into `impl/fs` directory. This makes for
better organisation of the source tree when we eventually have `impl/net`,
for network implementations in `Cogent`.