Support maximal-length encoded integers

[alexcrichton]

This came up during rustwasm/walrus#30, specifically rustwasm/walrus#30 (comment).

The use case here is that sometimes when dealing with leb128 you'll often have a scenario where a leb128 integer denotes how many bytes left in the region are part of a section or unit. When encoding, though, we often don't know the length of the section up-front, so a common trick is to do something like:

• For a u32 encoded as leb128, reserve 5 bytes of space
• Encode the entire section
• Encode the length of the section into the previously reserved 5 bytes of space

This encoding uses the maximal instead of minimal width, using "padding zero bytes" that look like 0x80 to ensure that leb128 eats up all 5 of the bytes reserved.

It'd be neat if this crate supported such a use case (it's sort of like Seek with writers), although I'd be fine just supporting it with slices for the time being!