Rust wrapper for SplinterDB

[etwest]

A rust wrapper for SplinterDB. Implements simple key/value operations by default and allows for more complicated update behavior with user defined rust functions.

The wrapper is an iterative improvement to a previous version created by @rosenhouse.

Changes from the previous version of the wrapper:

• Automatic splinter binding (splinterdb-sys) generation when building the rust library.
• More tests of the rust wrapper (splinterdb-rs).
• Support for new data_config layout and other changes to SplinterDB.
• Ability to callback to rust functions via the SplinterDB data_config. These callbacks are defined by the SdbRustDataFuncs trait.