Closes #414.

Converted some S3 classes to S7 in form.R and session.R.

Notable changes:

• The S3 implementation of rvest_field allowed for dots to be passed. I noted that the dots were only ever used to pass one (optional) argument, options; as such, I added options as a new property to the S7 implementation but did not include a way to pass the dots when creating a rvest_field. That is an easy fix if needed.
• Adjusted tests to match S7 syntax and the like.
• Bumped Roxygen to 7.3.2.

Issues

• The "basic session process works as expected" test fails locally. I get the same (incorrect) test output with the original S3 implementation cloned from Github though. I get Size: 821905 instead of Size: 20707 as the snapshot asserts. I think this is an error with the test?
• No documentation for any S7 pecularities.