Pretty error messages: fixes #6.

Phew, this ended up being a pretty complicated change!
The basic summary is that we now support the syntax
```
@jaxtyped(typechecker=typechecker)
def f(...): ...
```
and when using this, we now get pretty error messages about what went
wrong.

(
The old syntax, i.e.
```
@jaxtyped
@typechecker
def f(...): ...
```
is still supported, but doesn't give much information.
)

The internals of this do quite a lot of magic! In particular we
dynamically create quite a lot of functions and test the provided
arguments against their signatures. The overhead should still be
minimal under `jax.jit`, though.
(TODO: what's the overhead like in non-jit situations, e.g. PyTorch?
I've tried to minimise the overhead throughout just to be sure, but
perhaps PyTorch users should stick to the old syntax?)

docs/api/array.md

@@ -24,7 +24,7 @@ In addition some modifiers can be applied:
     `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"]`.
-- Prepend `?` to a dimension to indicate that its size can vary within a PyTree structure. (See [PyTree annotations](../pytree.md).)
+- Prepend `?` to a dimension to indicate that its size can vary within a PyTree structure. (See [PyTree annotations](../pytree/).)
 
 When using multiple modifiers, their order does not matter.