Support coercing float to int if strict=False - #619

This adds a path for coercing floats to ints if `strict=False`, provided
the float has an exact integral representation.

```
>>> import msgspec

>>> msgspec.convert(1.0, int, strict=False)
1

>>> msgspec.convert(1.5, int, strict=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: Expected `int`, got `float`
```