Implement Add on Option types

This will allow you to use the + operator to add together any two
Options, assuming that the contents of each Option likewise implement
+. So Some(4) + Some(1) == Some(5), and adding with None leaves the
other value unchanged.

This might be monoidic? I don't know what that word means!
auto merge of #5328 : bstrie/rust/optadd, r=graydon

This will allow you to use the `+` operator to add together any two
Options, assuming that the contents of each Option likewise implement
`+`. So Some(4) + Some(1) == Some(5), and adding with None leaves the
other value unchanged.

This might be monoidic? I don't know what that word means!