Improve code positions (match + extended assignments)

[Cameron-Low]

• ^match pattern now allowed within a code pos
• #cname can be used to select the appropriate sub-branch of a match
• ^lv<@ and ^lv<$ are now permited

[strub]

If you don't mind, making two PR (you make two things in that PR) would make more sense. (I'd like to use the PR for generating a changelog)

[Cameron-Low]

Removed the new print command from this PR.

[strub]

Could you add an example? And then, we'll be good to go.

[Cameron-Low]

I've added the test, and made minor changes to pp_codepos to account for the recent changes to main.