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.
a30d61b05a removed all of the trailing whitespace in doc/index.md.
Unfortunately, that trailing whitespace was actually markdown syntax for
line breaks.
