[Parser] Parse struct and array types (#4745)

Parse struct and array type definitions along with field names. Only the most
basic definitions are parsed for now; subtype definitions (both nominal
prototype and standard formats) and recursion groups are left to follow-on PRs.

Since there is no official standard for the text format for GC type definitions,
attempt to define a grammar that allows abbreviations that we already use
widely, such as making `(field ... )` optional except for named fields.

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