Added various parts of the well typedness checks

[mlaveaux]

Implemented so far:

• Check for syntactic empty sorts
• Perform initial name resolution on the sorts.
• Parse and add the rules for all the standard and basic sorts.
• Check for alias cycles.

[Copilot]

Pull request overview

This PR introduces an initial merc_typecheck crate that starts implementing well-typedness checks for data specifications (non-emptiness, name resolution, standard/basic sort rules, and alias-cycle detection), and extends the syntax AST/visitors to support resolved sort references and flattened function sorts.

Changes:

• Add merc_typecheck crate with name resolution, alias-cycle detection, non-emptiness checking, and standard/basic sort specification helpers.
• Extend merc_syntax sort AST with DefId, Resolved sort references, and FlattenedFunction, updating visitors/builders/display accordingly.
• Wire the new crate into the workspace and dependents; add small utilities/tests in merc_collections.

Reviewed changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
crates/typecheck/src/standard_sorts.rs	Generates/instantiates standard/basic sort data specs and helper replacements.
crates/typecheck/src/non_empty.rs	Adds syntactic non-emptiness checking and a regression test.
crates/typecheck/src/name_resolution.rs	Implements initial sort name resolution to DefIds.
crates/typecheck/src/lib.rs	Exposes the new typecheck modules.
crates/typecheck/src/is_well_typed.rs	Adds well-typedness checks and defines WellTypedError plus tests.
crates/typecheck/src/is_finite.rs	Adds a finiteness predicate for sorts.
crates/typecheck/src/data_specification.rs	Adds a DataSpecification::from_untyped pipeline (flatten, resolve, cycle-check, well-typed checks).
crates/typecheck/src/alias.rs	Adds alias-cycle detection via SCC decomposition plus tests.
crates/typecheck/Cargo.toml	Declares the new merc_typecheck crate.
crates/syntax/src/visitor.rs	Updates sort-expression visiting APIs and adds traversal support for new sort variants.
crates/syntax/src/syntax_tree_display.rs	Adds display for Resolved and FlattenedFunction.
crates/syntax/src/syntax_tree.rs	Renames DeclId → DefId and adds Resolved/FlattenedFunction to SortExpression.
crates/syntax/src/consume.rs	Minor parsing code style adjustment.
crates/syntax/src/builder.rs	Generalizes apply_sort_expression error type and supports new sort variants.
crates/sabre/Cargo.toml	Adds dependency on merc_typecheck.
crates/collections/src/indexed_set.rs	Adds IndexedSet::get_unchecked.
crates/collections/src/block_partition.rs	Updates docs, removes a debug assert in is_empty, and adjusts tests.
Cargo.toml	Adds crates/typecheck to workspace members and workspace dependencies.
Cargo.lock	Adds the new merc_typecheck package entry and dependency edges.

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