Added : as let shorthand

: is the first and only infix operator in FL, since it's being used in pretty
much the only context where an infix operator would be unambiguous. a:b c is
equivalent to let a b c.

    add1: (+ .1 1)      add1 4

Also, updated all the examples to use this new notation
Extended equality tests to pairs

Equality tests (eq, lt, gt) now apply to pairs as well; the first elements are
compared first, and if they are identical, the second elements are compared.
The empty list is less than every other list.
Added README