Show expected and actual length for length error

[GIorfindel]

This PR adds the actual and expected size for length errors. This is useful when an assert is performed on a large array or string without knowing the length of it.

[GIorfindel]

For example,
AssertionError: ["a","2","ZAR","ar","ar","zt","zt","zt","zt","zt","zt","et","tr","xdf","ar","ar","zre","zer","ERZ","45"] must have length of 3
becomes
AssertionError: ["a","2","ZAR","ar","ar","zt","zt","zt","zt","zt","zt","et","tr","xdf","ar","ar","zre","zer","ERZ","45"] must have length of 3. +expected, -actual : +3 -20
We don't need to count each item anymore to know the actual length of the array.