Remove incorrect assert in trans

As an optimization, once unboxed closures receive their environment by
value if it fits within the size of an `int`.  An assert in this code
path assumed that this would only occur if the environment had no more
than a single free variable in it, but multiple smaller free variables
can easily be packed into the space of an `int`, particularly if any
of them are 0-sized.  The assert can simply be removed.

Closes #18652
Add regression test for #18652
Remove unnecessary `.to_string()`s from `Vec.swap_remove`'s doc example.
Implement len() for EnumSet.
Renamed Extendable to Extend

In order to upgrade, simply rename the Extendable trait to Extend in
your code

Part of #18424

[breaking-change]
Implements Extend for EnumSet and LruCache

Part of #18424
auto merge of #18735 : utkarshkukreti/rust/remove-unnecessary-to_string-from-vec-docs, r=huonw

I don't think they're needed.
auto merge of #18740 : jbcrail/rust/implement-enum-set-len, r=alexcrichton

This commit adds the missing EnumSet method mentioned by @Gankro.

cc #18424
auto merge of #18475 : gamazeps/rust/toExtend, r=alexcrichton

Ensured that Extend & FromIterator are implemented for the libcollection.

Removed the fact that FromIterator had to be implemented in order to implement Extend, as it did not make sense for LruCache (it needs to be given a size and there are no Default for LruCache).

Changed the name from Extend to Extendable.

Part of #18424
auto merge of #18730 : bkoropoff/rust/issue-18652, r=eddyb

`FnOnce` environments that fit within an `int` are passed to the closure by value.  For some reason there was an assert that this would only happen if there were 1 or 0 free variables, but it can also happen if there are multiple variables that happen to fit.

Closes #18652