[DSLX] Path-update operator `:=` (mutation-free)

It's convenient to have a shorthand notation for "deeper path" updates, say to a struct that carries nuanced data from "tick" to "tick" (which ultimately is state since it iterates in time, even though we don't mutate it). I was thinking of this shorthand syntax -- it'd be convenient to do something like:

`s.data[foo][bar] := u32:42;`

which would basically do:

`let s = State{data: update(s.data, foo, update(s.data[foo], bar, u32:42)), ..s};`

as a desugaring so you don't need to write it all out. Note that this operator is rebinding the "outermost" referenced identifier (in the scope in which it's written).