Add parsing of timestamps

We add the parsing of timestamps written as strings in this MR.

• add the opam dependency coq-moment;
• use the function of coq-moment to parse timestamps at the rfc3339 format;
• customize the extraction to avoid a name clash on the Char module;
• bonus: install apt dependencies from opam.

Something for which I do not know what to do: Michelson accepts timestamps with space as a separator instead of T. This is technically out of the RFC3339 spec but often accepted in practice. For now, this MR only accepts the T separator.