Support struct-like variants in `parser.py`

Consider

pub enum VariantsShape {
    Single { index: VariantIdx },
    Multiple {
        tag: Scalar,
        tag_encoding: TagEncoding,
        tag_field: usize,
        variants: Vec<LayoutShape>,
    },
}

This can be modeled with proper parser support in K as follows:

syntax VariantsShape ::= variantsShapeSingle(VariantsShapeSingle)     [group(mir-enum), symbol(VariantsShape::Single)]
                       | variantsShapeMultiple(VariantsShapeMultiple) [group(mir-enum), symbol(VariantsShape::Multiple)]

syntax VariantsShapeSingle ::= mk(index: VariantIdx) [group(mir---index)]

syntax VariantsShapeMultiple ::= mk ( tag: Scalar
                                    , tagEncoding: TagEncoding
                                    , tagField: MIRInt
                                    , variants: LayoutShapes
                                    ) [group(mir---tag--tag-encoding--tag-field--variants)]

The ideal solution would be to eliminate the mk constructors, and model the struct-like variants directly, e.g.:

syntax VariantsShape ::= variantsShapeSingle(index: VariantIdx) [group(mir-enum---index), symbol(VariantsShape::Single)]
                       | variantsShapeMultiple ( tag: Scalar
                                               , tagEncoding: TagEncoding
                                               , tagField: MIRInt
                                               , variants: LayoutShapes
                                               ) [group(mir-enum---tag--tag-encoding--tag-field--variants), symbol(VariantsShape::Multiple)]

Start in parser.py and compare the existing handling of enum variants with the VariantsShape examples in the issue. Determine how the Single and Multiple fields map into the parser's K syntax, then verify that both struct-like forms are accepted and represented correctly. The ideal completion removes the intermediate mk constructors where possible.