Types like output(T), promise(T), and union(T_0, ..., T_N) should be
assignable from dynamic if they contain an element type that is
assignable from dynamic.
This allows consumers to use the following code to check if some type
behaves like the dynamic type w.r.t. conversions:
```
if t.AssignableFrom(model.DynamicType) {
}
```
Fixes#4703.
Add a rewriter that reifies implicit conversions into a call to the
`__convert` intrinsic. Code generators can recognize this intrinsic and
use it to generate appropriate conversion code.
Part of this work involves redesigning the type annotations system.
Annotations are now only applicable to opaque and object types. Instead
of inspecting annotations directly, code generators should use
`hcl2.GetSchemaForType` to extract the `schema.Type` for a `model.Type`.
These changes add a package for type checking and modeling HCL2
configurations. It is made up of three primary components:
1. A static type system
2. A semantic representation of HCL2 expressions and a binder from HCL2
native syntax to this representation
3. A semantic representation of HCL2 structural elements and binders
from HCL2 native syntax to this representation.
The type system is described in the "Extended Types" section of the
specification. The semantic representations of expressions and
structural elements are documented in their implementations.
