comptime slice oob check is off-by-one

[cyberthirst]

Version Information

• vyper Version (output of vyper --version): 14a4ca6

What's your issue about?

Per @trocher:

In Slice.fetch_call_return, the following bounds check contains an off-by-one error that makes it
less precise than it could be:

if start_literal is not None:
    if start_literal > arg_type.length:
        raise ArgumentException(f"slice out of bounds for {arg_type}", start_expr)
    if length_literal is not None and start_literal + length_literal > arg_type.length:
        raise ArgumentException(f"slice out of bounds for {arg_type}", node)

The condition start_literal > arg_type.length could be made more precise by checking
start_literal >= arg_type.length instead. This is because a slice is out of bounds when the
start index is equal to the length of the array, not just when it exceeds the length.