Reduce references to niche optimization

[djmitche]

Niche optimization is currently mentioned in three places:

• Enums (User-Defined Types, Day 1 Afternoon)
• Option (Standard Library Types, Day 2 Afternoon)
• Box (Smart Pointers, Day 3 Morning)

This is a tricky thing to get right, and it was just in the speaker notes in each place. #1820 will introduce a fuller explanation.

Fixes #1820.