Avoid excessive reallocations during item-bodies checking

Avoid excessive reallocations during item-bodies checking

When foldings Substs, we map over VecPerParamSpace instances using
EnumeratedItems which does not provide an accurate size_hint()
in its Iterator implementation. This leads to quite a large number or
reallocations. Providing a suitable size_hint() implementation reduces
the time spent in item-bodies checking quite a bit.

```
crate  | before | after | ~change
-------|-------------------------
core   |  7.28s | 5.44s |   -25%
std    |  2.07s | 1.88s |  -9.2%
syntax |  8.86s | 8.30s |  -6.3%
```