Instrument and emit stats on program RAM usage

[cratelyn]

Fastly's UI provides stats on the RAM usage of a Compute@Edge program running in production. Viceroy should emit information about the memory usage of a single instance execution, so that users can reason about the behavior of their program in production.

• see also Instrument and emit stats on program wall clock duration #76.
• see also Instrument and emit stats on program CPU time #78.