[Yhc] Proposal: Yhc Core file generation

[Neil Mitchell]

Hi,

There are shortly going to be 2 projects that use Yhc.Core
extensively, one of them even _not_ written by me! As such maybe now
is the time to start taking our Core output a bit more seriously - so
here are my proposals:

1) core=1 is the default for Scons, installed libraries contain .ycr files
 - Slower to compile the libraries (a bit) takes up more space (a few Mb)
 + useful for the Yhc.Core users

2) Make the -core plus lack of a .ycr file, or an old .ycr file, cause
a file to be rebuilt. At the moment it only has an effect on newly
built files.
 - more recompilation
 + but only for people who specify -core

3) Add a -corelink option, which implicitly issues -core, and after
all the .ycr files have been generated, pulls them together into one
big .yca file.
 + useful for Yhc.Core users

4) Add a -O option that calls Catch with the big .yca file after its
be generated.
 + optimised binaries :)
 + can default to GHC -O2 when catch is unavailable

In addition there will be another user of the Yhc.Core files, which
will be announced shortly, but for which steps 1 through 3 would be
really handy.

So basically, only 1) hurts anyone, and not by that much. Does this
seem fine to everyone?

Thanks

Neil