In the JS backend `toOpenArray` used `slice` (a copy), so writes through
a `var openArray` parameter silently vanished.
This emits `subarray` (a live shared-buffer view) for homogeneous
numeric arrays, otherwise such parameters are passed as a `{base, off,
len}` view that always aliases the caller's storage. Sliced seq/array
args become `{base, off, len}`, whole values `{base, off:0, len}`,
re-slices rebase.
Un-skips the JS guard in tests/openarray/topenarray.nim
Fixes#15952.
fixes#23321
In the function `mapType`, ptrs (tyPtr, tyVar, tyLent, tyRef)
are mapped into ctPtrToArray, the dereference of which is skipped
in the `genref`. We need to skip these ptrs in the function
`genOpenArraySlice`.
