Editorial: Consistify miscellaneous phrases

[h2oche]

I observed some phrases are written in inconsistent form:

• To extract the element from the list of length 1, other phrases use the sole element of.
• To check if the list is empty, other phrases use 'is empty' or 'is not empty'
• To reference some well-known objects, other phrases use active function object and the running execution context, respectively.
• To append an element to the end of list, other phrases use append _element_ to _list_.
• Removed unnecessary prefix 'the String' for denoting a string literal.

[michaelficarra]

The final commit here can be dropped in favour of #2711

[michaelficarra]

LGTM otherwise. This is great!

[h2oche]

@michaelficarra Oh, I see. I dropped it.

[bakkot]

Looks good to me except for the three uses of "append" for updating a Set.

[michaelficarra]

@bakkot @h2oche I've gone ahead and just made the requested change. Should be ready to merge now.