Fix warnings and documentation for "at-most" constructs

[tahina-pro]

This PR is a partial solution to #124 :

• It fixes the deprecation warning raised by things like T t[<= n], now directing to use [:byte-size-single-element-array-at-most instead of the non-existing [:byte-size-at-most
• It fixes the documentation wrt. the semantics of T [:byte-size-single-element-array-at-most n] : we validate the format of T and check that it fits within n bytes, and then we always consume exactly n bytes by advancing the position of the parser. So, this in effect allows one to encode an element T padded out to n bytes.
• It turns the warning raised by T x[n] with sizeof(T) <> 1 into an error.