schema/lang: Rename Traversal* to Reference*

[radeksimko]

Depends on

• decoder/schema: Remove old (unused) logic related to legacy expressions #267

Context

As part of #185 and some other related PRs we started using a slightly different naming convention (traversal -> reference essentially).

We used the term "reference" in other places before (there's whole package reference), so it makes sense to align the rest of the codebase and get rid of "traversal", which is usually only how we refer to references in HCL internals. The aim is to make the public API friendlier to developers who have to build the schema.

Proposal

• Rename all existing types/functions which involve "traversal" to "reference".
• Update any references in terraform-schema
• Update any references in terraform-ls