Corresponding upstream change:
golang/go@23832ba.

Our implementation is not, in fact, optimized, but at least it is
correct.

The JavaScript typed array that backs Go array must be shrunk to match
the expected Go type. If this is not done, copying the converted array
into an array of the same (expected) size causes a "source too large"
runtime exception.

The optimization this package implements relies heavily on finalizers,
which GopherJS doesn't support. As such it would create a memory leak
rather than an actual optimization. Until we decide that we can use
WeakMap API, this package can't be safely supported.

The only current use of this package is in the `net/netip` package and
this commit adds overlays that replace uses of the intern package with
direct use of strings.

In GopherJS this function is a no-op, since we don't have access to
low-level memory management. We also have to disable the corresponding
test, since it uses unsafe features we don't support.

Apparently, under V8 division by zero literal and by zero assigned to a
variable yields bitwise-different values of NaN, which broke some tests
in `internal/fuzz`. Using an actual NaN value from the JavaScript
runtime fixes the issue.

Isolated example: https://jsfiddle.net/j3rtaz0o/1/. Note that Firefox
doesn't seem to exhibit this issue.

This package uses generics in its tests, which we currently don't
support. This causes the compiler to crash.