ApproachFrom should be able to name a direction in the complex plane

Split out of #596, where @Happypig375 noted: *"we should also add some way to represent a limit direction in the complex plane."*

`ApproachFrom` is `Left`, `Right`, `BothSides`. On the real line that is the whole story. In the complex plane it is not: approaching 0 along the negative reals is one ray out of infinitely many, so `lim x->0- ln(x) = -oo` is really a *directional* limit that happens to be spelled "from the left".

This matters beyond notation. Under a complex codomain, "from the left" and "from the right" are two arbitrary rays, and there is no reason the two of them agreeing should say anything about the limit as a whole -- which is part of why the promotion in the sibling issue has to be tied to the codomain rather than applied generally.

## Shape

`ApproachFrom` grows a way to name a direction, e.g. a limit along `e^(i*theta)`, with `Left` and `Right` staying as the `theta = pi` and `theta = 0` cases so nothing existing changes. `ComputeLimitImpl` already maps a finite destination to infinity by substituting `dest +- 1/x`; the directional case is `dest + e^(i*theta)/x`, so the substitution generalises cleanly.

It is an enum that has to grow rather than a rule that has to change, which is why this is filed separately -- it is a public API change and wants its own discussion.

Start by locating the public ApproachFrom enum and reading ComputeLimitImpl, then review the context in sibling issue #596. Define how the directional case represents e^(i*theta)/x and verify that Left and Right retain their existing meanings while directional limits use the generalized substitution.

Written by the indexing model from the issue text.