Support size-shrinking value and reference transmutes

[joshlf]

Support transmutations that allow shrinking the size of the value or referent.

This follows up on the work done in #1817.

Progress

• Support value transmutations (transmute!) that permit shrinking size
  • [macros] Support shrinking value transmutes #2581
• Support reference transmutations (transmute_{ref,mut}!) that permit shrinking size
  • Generalize SizeEq to SizeFrom, support shrinking #2580
  • [macros] Support shrinking reference transmutes #2487
• Support fallible value transmutations (try_transmute!) that permit shrinking size
• Support fallible reference transmutations (try_transmute_{ref,mut}!) that permit shrinking size