Make a jit chain that can reuse and share elements between forms

[Martin Sandve Alnæs]

A large part of the jit process is code generation, swigging and compiling of finite elements. These are mostly the same from form to form. There's likely to be a major time saving in the jit part of dolfin programs if this can be exploited.

[Martin Sandve Alnæs]

Duplicate of #90.