compiler: bulk rename stdgum to libgum in all places

impl/fs/ext2: Pass preprocessor flags to Cogent's cpphs instance.

This allows us to, when we're running in verification mode, define a
`-DVERIFICATION' symbol, which could (e.g.,) hide expressions that do
not produce verifiable code.

Improve ext2 makefile.

New TAGS target, to build tags for emacs.
Use static assignment instead of binding for some variables, to
improve performance.

libgum: reorganise cogent log functions.

Used by the various modules, the logging functions had some missing
pieces. This patch cleans it up.

quickcheck-readpage: trying shallow embdg

[skip lemma]

compiler+fs: fix naming of generated files

[skip lemma]

impl: Use gcc headers from the correct version.

Remove the old style debug statements.

Cogent earlier had statements starting with an `_` which the compiler
would ignore in debug mode and this behaviour has changed. This commit
removes those statements and is the first phase of using the
C preprocessor style debug conditionals.

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.

[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`.