This reverts commit cb5a6be41ae0efc30d0b59a41b0763db966e5bf4.

I deeply apologize for the churn.

This change is problematic given that we do not have ranged integers
(yet? see #3806).

In the meantime, this type needs to be `usize`, matching the length and
index types for all std lib data structures.

Users who want to save memory should not use heap-allocated BoundedArray
values, since it is inherently memory-inefficient. Use a different
memory layout instead.

If #3806 is accepted and implemented, the length value can become an
integer with the appropriate range, without the footgun. If that
proposal is not accepted, len type will remain a usize.