impl/fs/ext2: Make log messages disappear under verification.

The AutoCorres C parser chokes on string literals. Instead, with
`-DVERIFICATION' set, all the calls to <gum/common/log.cogent> logging
functions disappear in a suffusion of C preprocessor tokens.

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.

ext2: typechecks again with upcast

ext2: new tc passed!!!

ext2: wip - porting to new type system

[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`.