Move separation-logic basics into coqutil.

[jadephilipoom]

Follow-up to mit-plv/coqutil#124

• First commit bumps coqutil to catch the copied files
• Second commit removes the files from bedrock2 and replaces references to them with references to coqutil.

To minimize the changes I kept the files in bedrock2 but made them simply one-line exports of the new locations in coqutil.

[jadephilipoom]

CC @andres-erbsen

[andres-erbsen]

build (master) Expected — Waiting for status to be reported