Finish Varargs PR; reduced API surface

Since this is a rarely used feature, a rich encapsulated API for error handling is likely not needed.
Most users do not explicitly differentiate error variants, having a descriptive Display impl is usually more important.

Supports only 4 expressions for length checks: a, a.., ..=b, a..=b.
These are almost always literals and exclusive bounds are likely error prone in this context.
Additionally, check_length() with the full range '..' makes no sense, so it's not supported.

Author: Bromeon