Add SSA to the Tires syntax

[ELLIOTTCABLE]

The Tires syntax is restrictive: there's only a subset of all possible dependency graphs that can be expressed in that way. (In fact, it's more of a tree than a graph.) I've previously considered having a completely basic SSA syntax for dependency declaration; I now suspect this can be combined with Tires (such that individual lines of Tires-style expressive encoding can be the ‘body’ of SSA lines; and SSA names can be utilized in such Tires-style encoded lines.)

In this way, any Tires-encoded graph can be deconstructed down to simple SSA for analysis; and, to boot, parts of a desired graph that cannot be Tires-encoded can instead be encoded in the same document with more verbose SSA.