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%
```
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