Currently, the rust build system in Scylla creates a separate
static library for each incuded rust package. This could cause
duplicate symbol issues when linking against multiple libraries
compiled from rust.
This issue is fixed in this patch by creating a single static library
to link against, which combines all rust packages implemented in
Scylla.
The Cargo.lock for the combined build is now tracked, so that all
users of the same scylla version also use the same versions of
imported rust modules.
Additionally, the rust package implementation and usage
docs are modified to be compatible with the build changes.
This patch also adds a new header file 'rust/cxx.hh' that contains
definitions of additional rust types available in c++.
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