Fix large-dates ambiguity by creating a repr:four modifier

[dennisorlando]

This is linked to #683

This PR creates a new [year] modifier, i.e. repr:four, which represents a full year which is forced to be 4 digits long. This is different from the default repr:full, which has a variable amount of digits depending if the large-dates feature is enabled, thus causing parsing ambiguities that might produce errors in crates who depend oh time.rs.

Tests have still to be written for this PR, thus it's not ready to be merged, but I opened this pull request anyways so that someone else might write them. Namings and comments might also need to be discussed.

(I'll make a rebase of the commits when this PR is ready to be merged)