Embed sqlite amalgamation v3.40.0 source code

This patch was authored by @Austinpayne and released by @gwynne.

This PR removes dependence on a system libsqlite3 by embedding the amalgamation (single source file) release of sqlite3 in vendored form. The included source code is fetched from https://sqlite.org/download.html. (#35)

Projects which include logic in their Dockerfile and/or CI workflows to ensure that the libsqlite3-dev package (or equivalent) is installed on the build image can now safely remove it, but are not required to do so. No additional actions are necessary to take advantage of any additional features supported by the embedded library; SQLKit and Fluent will detect their availability automatically when possible (most notably UPSERTsupport).

Closes #32