Auto merge of #42417 - eddyb:separate-fn-sig, r=nikomatsakis

Don't drag function signatures along function item types.

This PR separates the signature of a function from the "function item type" (`TyFnDef`), leaving only the `DefId` and parameter `Substs`, making them even more like (captureless) closure types.

The motivation for this change is reducing typesystem complexity, and its consequences:
* operating on the signature instead of just the parameters was less efficient
  * specifically, signatures can easily add several levels of depth on top of the parameter types
  * and the signatured were always substituted and normalized, so typically even more complex
* it was *the only* type that was *both* nominal (identity) and structural (signature)
  * harder to model in Chalk than either a purely nominal or structural type
  * subtyping worked on the signature but parameters were always invariant
  * call type-checking was transforming signatures but keeping the nominal half intact
  * the signature could therefore get out of sync during type inference in several ways

That last point comes with a `[breaking-change]`, because functions with `'static` in their return types will now *not* be as usable as if they were using lifetime parameters instead:
```rust
// Will cause lifetime mismatch in main after this PR.
fn bar() -> &'static str { "bar" }
// Will continue to work fine, as every use can choose its own lifetime.
fn bar<'a>() -> &'a str { "bar" }

fn main() {
    let s = String::from("foo");
    Some(&s[..]).unwrap_or_else(bar);
}
```

r? @nikomatsakis

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