Simplify implicit cast ctors for vector & matrix.

[csyonghe]

Closes #6405, and helps with #6358.

This pull request introduces a new builtin type constraint to express "T is implicitly convertible to U", and use this constraint to turn many vector/matrix ctor overloads into a single generic ctor.

Previously, our core module defines:

vector<int, N>.__init(vector<uint, N>);
vector<int, N>.__init(vector<float, N>);
vector<int, N>.__init(vector<bool, N>);
...

These overloads results in large core module size, and makes overload resolution slower than it needs to be.

With this cleanup, we are now expressing this with a generic ctor:

extension<T, int N> vector<T, N>
{
     __init<U>(vector<U,N> v) where T(U) implicit;
}

The main change here is to implicit the new T(U) implicit type constraint (TypeCoercionConstraint) in a minimal form that is just enough to express the core module functionality.

In the future we can expand the language to fully support this new type of constraint for user code. For now the implementation is very limited in that we are cutting a lot of corners by not forming any real witness object to represent how the constraint can be satisfied, and the implementation is not functional for anything but the core module use.

Summary from copilot attached below.

Type Coercion and Conversion Enhancements:

• Added TypeCoercionWitness class and corresponding methods to handle type coercion witnesses in slang-ast-val.h and slang-ast-val.cpp. [1] [2]
• Introduced TypeCoercionConstraintDecl class in slang-ast-decl.h to represent type coercion constraints.
• Implemented getTypeCoercionWitness method in ASTBuilder class to create or retrieve type coercion witnesses in slang-ast-builder.cpp and slang-ast-builder.h. [1] [2]
• Updated getImplicitConversionCostWithKnownArg and _coerce methods to handle type coercion constraints in slang-check-conversion.cpp. [1] [2] [3]
• Added visitTypeCoercionConstraintDecl method to SemanticsDeclHeaderVisitor for validating type coercion constraints in slang-check-decl.cpp. [1] [2]

IR Instructions and Handling:

• Introduced BuiltinCast IR instruction in slang-ir-inst-defs.h and updated IRBuilder to handle it in slang-ir-insts.h and slang-ir.cpp. [1] [2] [3]
• Enhanced peephole optimization to handle BuiltinCast in slang-ir-peephole.cpp.

Miscellaneous:

• Removed outdated vector extension code and introduced new generic vector and matrix extensions in core.meta.slang. [1] [2]
• Added a new conversion cost kConversionCost_TypeCoercionConstraint and related constants in slang-ast-support-types.h.
• Updated ImplicitConversionModifier to set default values for conversion cost and kind in slang-ast-modifier.h.
• Modified overload candidate comparison to prefer implicit conversions in slang-check-overload.cpp.
• Adjusted constraint system solving to verify type coercion constraints in slang-check-constraint.cpp. [1] [2]

These changes collectively enhance the type coercion and conversion mechanisms, improve IR handling, and streamline the codebase.

[jkwak-work]

Looks good to me.

The copilot summary on the PR description isn't very helpful because it describes trivial changes explicitly too much.