Some improvements (#77)

* Various improvements.

- Added support for functions in symbolic dimensions, e.g. "min(foo,bar)", which were previously disallowed due to the presence of a comma. (#51)
- Added support for adding ignored names to dimensions, e.g. "cols=4". (#76)

* Now works with Python 3.10 A | B union types.

API.md

@@ -17,6 +17,7 @@ In addition some modifiers can be applied:
 - Prepend `*` to a dimension to indicate that it can match multiple axes, e.g. `"*batch c h w"` will match zero or more batch axes.
 - Prepend `#` to a dimension to indicate that it can be that size *or* equal to one -- i.e. broadcasting is acceptable, e.g. `def add(x: Float[Array, "#foo"], y: Float[Array, "#foo"]) -> Float[Array, "#foo"]`.
 - Prepend `_` to a dimension to disable any runtime checking of that dimension (so that it can be used just as documentation). This can also be used as just `_` on its own: e.g. `"b c _ _"`.
+- Documentation-only names (i.e. they're ignored by jaxtyping) can be handled by prepending a name followed by `=` e.g. `Float[Array, "rows=4 cols=3"]`.
 
 When using multiple modifiers, their order does not matter.