Introspection (part 6) (#113)

Following on from #112, this PR adds a `reflect`-like API for inspecting
durable coroutine state.

I've opted for a reflect-like API — rather than using `reflect.Type` and
`reflect.StructField` directly — because it's not currently possible to
map serialized named types back into a `reflect.Type` that has the same
properties and same identity. The serialization layer handles this
problem by tracking the offset of a `reflect.Type` in memory relative to
a known anchor point (see
[1](https://github.com/stealthrocket/coroutine/blob/f768d76b2a41ff2256ca7f049cb1fb457a077149/types/unsafe.go#L66-L84)
and
[2](https://github.com/stealthrocket/coroutine/blob/f768d76b2a41ff2256ca7f049cb1fb457a077149/types/types.go#L50-L52))
. This means that it's only possible to get the associated
`reflect.Type` values in the build that generated the state. It's
possible to create `reflect.Type`'s that are somewhat equivalent (e.g.
via `reflect.StructOf([]reflect.StructField)`), but it's not possible to
set certain properties such as the name or package path. I opted to use
a custom API rather than return incomplete/invalid information.

A notable omission and the subject of a follow-up PR is the ability to
walk the object graph. With this PR we can inspect the regions (the
length and type) but we can't unpack the encoded data within them.