handle: move the base_uri field to rust

In order to be able to return a const char pointer, we store the
base url also as a CString
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fedora test

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manual #181013
allowed to fail manual
clippy
passed #181012
rustfmt

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passed #181014
tarball

00:10:19

 
  Cross Distro
passed #181017
debian_i386:testing

00:07:59

passed #181016
debian:testing

00:07:22

passed #181015
fedora:rawhide

00:07:13