refactor!: migrate from thiserror to snafu

[lgiussan]

Rationale for this change

See spaceandtimelabs#174 for rationale

What changes are included in this PR?

NOTE: the individual commits may be good to review individually.

• All error enum variants consisting of tuple structs are transformed into named structs. This is necessary because snafu does not support tuple structs.
• Every #[derive(Error)] is substituted with the analogous #[derive(Snafu)]. In particular:
  • #[error(...)] attributes are substituted with equivalent #[snafu(display(...))] attributes
  • #[error(transparent)] attributes are substituted with equivalent #[snafu(transparent)] attributes (which also derive the corresponding From implementation)
  • For ConversionError::TimestampConversionError, the #[snafu(context(false), display(...))] attribute is used for deriving a From implementation, and at the same time maintain the custom error message
• A std feature is introduced for the proof-of-sql crate, which in turns activates the snafu/std feature. The std feature is required for the posql_db example, because Clap relies on the std::error::Error trait.
• thiserror still appears in the dependency tree (cargo tree -i thiserror), but only as a transitive dependency (via blitzar, and dev-dependencies)

Are these changes tested?

Yes. This PR is a refactoring, and all existing tests pass.