There's a lot of stuff wrong with the representation of these types:
TyFnDef doesn't actually uniquely identify a function, TyFnPtr is used to
represent method calls, TyFnDef in the sub-expression of a cast isn't
correctly reified, and probably some other stuff I haven't discovered yet.
Splitting them seems like the right first step, though.

cf. https://doc.rust-lang.org/error-index.html#E0082

Fixes #32004

+ some cleanup in rustdoc

This commit is the result of the FCPs ending for the 1.8 release cycle for both
the libs and the lang suteams. The full list of changes are:

Stabilized

* `braced_empty_structs`
* `augmented_assignments`
* `str::encode_utf16` - renamed from `utf16_units`
* `str::EncodeUtf16` - renamed from `Utf16Units`
* `Ref::map`
* `RefMut::map`
* `ptr::drop_in_place`
* `time::Instant`
* `time::SystemTime`
* `{Instant,SystemTime}::now`
* `{Instant,SystemTime}::duration_since` - renamed from `duration_from_earlier`
* `{Instant,SystemTime}::elapsed`
* Various `Add`/`Sub` impls for `Time` and `SystemTime`
* `SystemTimeError`
* `SystemTimeError::duration`
* Various impls for `SystemTimeError`
* `UNIX_EPOCH`
* `ops::{Add,Sub,Mul,Div,Rem,BitAnd,BitOr,BitXor,Shl,Shr}Assign`

Deprecated

* Scoped TLS (the `scoped_thread_local!` macro)
* `Ref::filter_map`
* `RefMut::filter_map`
* `RwLockReadGuard::map`
* `RwLockWriteGuard::map`
* `Condvar::wait_timeout_with`

Closes #27714
Closes #27715
Closes #27746
Closes #27748
Closes #27908
Closes #29866

The range desugaring does not use the lang items. Hence I did not add
lang items for inclusive ranges. This cleanup commit removes the old
unused ones as well.

Whether the desugaring _should_ use lang items is another question:
see #30809. But if we decide on a strategy there we can add back these
lang items, and new ones for inclusive ranges.

For stage0 we need to keep the attributes as the lang items still exist
even if they are never used.

This is surprisingly not a breaking change. Unused #[lang] attributes do
not even trigger a lint (see #30881).

A whole bunch of stuff gets folded into struct handling! Plus, removes
an ugly hack from trans and accidentally fixes a bug with constructing
ranges from references (see later commits with tests).

Change import a trait suggestion from:

   help: candidate #1: use `std::io::Write`

to

   help: candidate #1: `use std::io::Write`

so that the code can be copied directly.

Fixes #31504

rather than being implicit quasi-subtyping. Nothing good can come out
of quasi-subtyping.

be more fine-grained, fixing the `dep-graph-struct-signature`
test.

These describe the structure of all our crate dependencies.