New infrastructure: tracking changes in files and values

A FileMonitor is an abstraction for monitoring the status of files,
as well as changes in an in-memory value. Files can be tracked by file
modification time, or mod time plus content. We can also track files
that are expected not to exist (to help implement search paths). We can
also have file globs.

The abstraction is useful for re-running actions when input files or
values change. This pattern is captured by the Rebuild monad.

This adds a dependency on the hashable package (used by
unordered-containers). If this is a problem we can extract just the hash
function we need.

This is not used yet, so there's a temporary import just to make sure it
gets compiled.

Re-export getInstalledPackagesMonitorFiles from Cabal lib and add
getSourcePackagesMonitorFiles locally to D.C.IndexUtils.

These are for tracking changes to these bits of the environment, so that
it's possible for us to recompute things that depend on them.

Require QuickCheck >= 2.7

The tests use '===' and 'counterexample', which were added in QuickCheck-2.7.

It's unneeded. C files use 'defined(_WIN32)' and Haskell files use
'mingw32_HOST_OS'.

simplify the conditional to defined(_WIN32) && !defined(__CYGWIN__)

Miscellaneous improvements to cabal-install test suite

cbits: WIN32 guard for wider range of compilers

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

For some weird reason we need to call 'git status' before 'git diff-files' for
the latter to work correctly on Travis.

Solver: support setup deps for --allow-newer

Always use "cabal" as the program name when generating the man page.

The man page previously listed commands like "cabal.exe install" on Windows,
which was unnecessary.

[ci skip]

The "allow newer" feature works by transforming the package descriptions
to adjust the dependencies on certain packages. This transformation was
not being applied to the dependencies of custom setup scripts.

More nix-local-build merging: solver and setup wrapper features

And add a TODO describing a way to clarify the policy stuff.

Use 'gen-extra-source-files.sh' for cabal-install as well.

Allow 'user-config init' to use CABAL_CONFIG.

Typo in Hacking.md

An extra `from` was present