invertible ring elements

[Alizter]

In this PR we:

• Define notions of left and right invertibility of ring elements and define invertible elements as those which are both left and right invertible.
• We prove basic uniqueness properties of inverses.
• We prove basic closure properties of invertibility.
• We define the group of units of a ring.

[Alizter]

@jdchristensen I've done the changes you suggested and will merge when this is green.