Introduce PlaceTypeBuilder to simplify PlaceType construction

[coord-e]

It is also used to construct Assumption; I couldn't come up with a better name.

[Copilot]

Pull Request Overview

This PR introduces a PlaceTypeBuilder to streamline the construction of PlaceType and Assumption objects. The builder pattern helps manage existential variable indices properly when combining multiple PlaceTypes and RefinedTypes into a single one.

Key changes:

• Introduces PlaceTypeBuilder with methods for subsuming types and managing formulas
• Refactors existing PlaceType methods to use the builder pattern
• Simplifies code in multiple locations by replacing manual existential variable management

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/refine/env.rs	Adds PlaceTypeBuilder struct and refactors PlaceType methods to use it
src/refine.rs	Exports the new PlaceTypeBuilder
src/analyze/basic_block.rs	Updates code to use PlaceTypeBuilder instead of deprecated PlaceType methods

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