Ambiguous byte order semantics for "container" data types (namely, `struct`)

[d-v-b]

this fix reveals a flaw in our HasEndianness type: although the struct data type "has" endianness in the sense that it contains data types with byte order semantics, it doesn't have a literal .endianness attribute. we might need to formalize the concept of "contains other data types" to make it clear that any data type checks have to check the contained data types

Originally posted by @d-v-b in #4142 (comment)