ast/hir: Rename field-related structures

[petrochenkov]

I always forget what ast::Field and ast::StructField mean despite working with AST for long time, so this PR changes the naming to less confusing and more consistent.

• StructField -> FieldDef ("field definition")
• Field -> ExprField ("expression field", not "field expression")
• FieldPat -> PatField ("pattern field", not "field pattern")

Various visiting and other methods working with the fields are renamed correspondingly too.

The second commit reduces the size of ExprKind by boxing fields of ExprKind::Struct in preparation for #80080.

[rust-highfive]

r? @lcnr

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[joshtriplett]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit d1522b3 with merge 4151b69442450bfead61718d7fa73ffd0a778bf7...

[bors]

☀️ Try build successful - checks-actions
Build commit: 4151b69442450bfead61718d7fa73ffd0a778bf7 (4151b69442450bfead61718d7fa73ffd0a778bf7)

[rust-timer]

Queued 4151b69442450bfead61718d7fa73ffd0a778bf7 with parent 195ad48, future comparison URL.

[rust-timer]

Finished benchmarking try commit (4151b69442450bfead61718d7fa73ffd0a778bf7): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[lcnr]

@bors r+

[bors]

📌 Commit d1522b3 has been approved by lcnr

[bors]

⌛ Testing commit d1522b3 with merge b4adc21...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing b4adc21 to master...