build: Require Guile >= 2.0.9.

build: Require Guile >= 2.0.9.

* configure.ac: Bump requirement to 2.0.9.
* doc/guix.texi (Requirements): Adjust accordingly.
* README (Requirements): Likewise.
* build-aux/download.scm: Remove workaround for <http://bugs.gnu.org/13095>.
* guix/build/download.scm: Likewise.
(http-fetch)[post-2.0.7?]: Remove.  Remove conditional code for not
POST-2.0.7?.
* guix/http-client.scm:  Remove workaround for <http://bugs.gnu.org/13095>.
(http-fetch)[post-2.0.7?]: Remove.  Remove conditional code for not
POST-2.0.7?.
* guix/serialization.scm (read-latin1-string): Remove mention of 2.0.9.
* tests/nar.scm: Use (ice-9 control).
(let/ec): Remove.