Support ClassVar on Struct types - #281

Previously this was unsupported.

Postponed annotation evaluations are a bit of a tricky issue here. We
currently don't parse type annotations at import time (we do so lazily
on the first decode call). This is nice for two reasons:

- It keeps import times quick. Especially for CLI applications where not
  all struct types will be decoded in a single invocation, it pays off
  to avoid parsing types unless needed.
- It plays better with recursive structures, since we won't ever parse a
  type that isn't fully defined.

However, we do need to know which attributes are/aren't fields at Struct
type definition time, which means we'll have to detect if an attribute
is a `ClassVar` *without* eval-ing the annotation string. This is a bit
tricky to get right (enough), without compromising on performance.

Our solution is to only accept `ClassVar` annotations of the following
forms:

- `ClassVar` and `ClassVar[<type>]`
- `typing.ClassVar` and `typing.ClassVar[<type>]`

Importing either `ClassVar` or `typing` under an alias won't be detected
with postponed annotations. I did a quick survey of projects and
couldn't find a use of `ClassVar` that didn't fall into one of these
cases, so I think we're fine here.

Note that this doesn't rely solely on string comparisons, that's just a
fast-filter to discard the 99% of fields that aren't class variables. So
the following annotation is properly detected to not be a class
variable:

```python
from msgspec import Struct

ClassVar = list

class Example(Struct):
    x: ClassVar[int]  # msgspec can tell that this isn't a ClassVar
```

With this optimization, `ClassVar` detection has a negligible effect on
struct import performance, while still supporting all the common cases.