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.
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