Add Connection::release_memory method

[nvzqz]

This is behind a new release_memory feature flag. It simply calls sqlite3_db_release_memory on the raw handle.

Open questions:

• How should we expose sqlite3_release_memory?
• Can we add #[doc(alias = "sqlite3_db_release_memory")]? It would require at least Rust 1.48. This can perhaps be its own issue/PR.
• Should types with exclusive access to Connection expose this operation? Or expose the underlying connection reference?

[codecov]

Codecov Report

Merging #1164 (ecbb934) into master (7fb2d37) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1164      +/-   ##
==========================================
- Coverage   78.90%   78.85%   -0.06%     
==========================================
  Files          48       48              
  Lines        6020     6024       +4     
==========================================
  Hits         4750     4750              
- Misses       1270     1274       +4     

Impacted Files	Coverage Δ
src/lib.rs	89.32% <0.00%> (-0.61%)	⬇️

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[thomcc]

LGTM