Introduce new Constant

[JDBetteridge]

Description

This PR is dependent on all of COFFEE being removed first.

Associated Pull Requests:

• 701
• 294
• 98
• After Firedrake is merged this can go in also: 51

Fixes Issues:

• Maybe

Checklist for author:

• I have linted the codebase (make lint in the firedrake source directory).
• My changes generate no new warnings.
• All of my functions and classes have appropriate docstrings.
• I have commented my code where its purpose may be unclear.
• I have included and updated any relevant documentation.
• Documentation builds locally (make linkcheck; make html; make latexpdf in firedrake/docs directory)
• I have added tests specific to the issues fixed in this PR.
• I have added tests that exercise the new functionality I have introduced
• Tests pass locally (pytest tests in the firedrake source directory) (useful, but not essential if you don't have suitable hardware).
• I have performed a self-review of my own code using the below guidelines.

Checklist for reviewer:

• Docstrings present
• New tests present
• Code correctly commented
• No bad "code smells"
• No issues in parallel
• No CI issues (excessive parallelism/memory usage/time/warnings generated)
• Upstream/dependent branches and PRs are ready

[connorjward]

Same question about value.

[JDBetteridge]

The assign needs to happen for annotation to occur. We can get away with not zeroing the dat, but we need to assign to create an assign block.