add GHCJS fields and generalise flag handling

path templates.

CompilerInfo contains more information about the compiler than
CompilerId, which just stores the flavour and version. In particular,
CompilerInfo knows an AbiTag, which can be used to tell binary
incompatible results from the same compiler apart, and detailed
information about supported languages and extensions.

Some fields in CompilerInfo may be left unknown (Nothing). This can
be used in the future to allow partially resolving configurations
based on supported languages or extensions.

This patch adds the js-sources field, for JavaScript source code
that should be included in compiled programs, similar to c-sources
for other targets.

It also adds ghcjs-options, ghcjs-shared-options and ghcjs-prof-options
and makes the underlying code for options easier to extend.

Fixes #2258.

Use text header for persistent build config

getConfigStateFile now throws meaningful exceptions which are caught by
tryGetConfigStateFile and friends, which are allowed to propagate,
rather than just calling 'die'. If the LocalBuildInfo was generated by
an older version of Cabal, an exception is still generated, but the
LocalBuildInfo is included if it is recoverable. This feature is used to
reduce code duplication between the library and the test suite.

Add delete-source to cabal sandbox help.

Fixes #2207.

GHC has been changed to guarantee producing valid object files for
signatures (they will usually be empty).

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

fix bootstrap sandbox help documentation

Preliminary support for compiling signature files, using --instantiate-with

There's no chrome here, but some of the guts for Cabal supporting compiling
signatures.  The key UI is a new --instantiate-with flag for ./Setup (no support
cabal-install side!) which properly modifies the package key, calculates extra
hole dependencies for a package, and ensures an appropriately
translated -sig-of is passed to GHC.  The UI here is supremely user-unfriendly:
the intent is that users will use cabal-install to calculate these parameters
for them.

ToDo: Cabal's inconsistency check in ./Setup needs to be adjusted to be
less zealous.

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

add data-dir to InstalledPackageInfo

add ISC license

the datadir location for the inplace InstallDirs was pointing to the top directory
of the package instead of the directory containing the data. this has been fixed.
since templates have already been expanded at this point, datasubdir raises an
exception an cannot be used in generalInstalledPackageInfo. therefore the
full path is stored in datadir.

 SavedConfig Monoid instance: make list fields work like Flags.

Fixes #1884. See code comments for more details.

`deepseq-1.4.0.0`'s major change is in the default `rnf` method
implementation (see haskell/deepseq#1 for details). However, `Cabal` isn't affected
by that, and happily compiles against the current (yet unreleased) `deepseq-1.4.0.0`.

(approved by @dcoutts)

Prettify code to ensure compilation with -XNoMonomorphismRestriction

docs: change detailed-1.0 to detailed-0.9

It's not clear to me that there even is a detailed-1.0. I searched around the repository for something about it and I don't see anything.

Fixes #2211.