In line with the dissolution of libextra - #8784 - moves arena to its own library libarena.
Changes based on PR #11787. Updates .gitignore to ignore doc/arena.
This is hopefully the beginning of the long-awaited dissolution of libextra.
Using the newly created build infrastructure for building libraries, I decided
to move the first module out of libextra.
While not being a particularly meaty module in and of itself, the flate module
is required by rustc and additionally has a native C dependency. I was able to
very easily split out the C dependency from rustrt, update librustc, and
magically everything gets installed to the right locations and built
automatically.
This is meant to be a proof-of-concept commit to how easy it is to remove
modules from libextra now. I didn't put any effort into modernizing the
interface of libflate or updating it other than to remove the one glob import it
had.
Before this patch, if you wanted to add a crate to the build system you had to
change about 100 lines across 8 separate makefiles. This is highly error prone
and opaque to all but a few. This refactoring is targeted at consolidating this
effort so adding a new crate adds one line in one file in a way that everyone
can understand it.
