Small tidy ups

[connec]

• 4a9a99c refactor: use in-memory sqlite for tests and examples

  This avoids having to manage temporary files, and lets us drop the
  tempfile dev dependency.

• 2e62b8c refactor!: replace sqlx_impls macro with blanket impl

  Less macros, less problems? This also makes the crate easier to use with
  other databases (e.g. custom ones).

  This could probably technically be a breaking change in some situations,
  e.g. if a specific implementation was provided for a custom
  sqlx::Database type.

  BREAKING CHANGE: The blanket impl will conflict with any explicit impls
  of sqlx::Executor on Tx<_>.

• f6f43db refactor: convert Tx from a tuple struct

  Named fields are generally a bit clearer for structs with multiple
  fields.