Vec::with_capacity(capacity) panics / aborts when the capacity is too large.

This is a safer alternative which reports Error using tryreserveexact

``` //let's pretend this is an arbitrary number read from a broken file let numberfromfile : usize = 100000000000000;

//try to create a 100TB Vec let result : Result, > = vecwithcapacitysafe(numberfromfile);

//An error is reported assert!(result.is_err()); ```

changelog

0.2

Added some documentations.

Changed implementation to tryreserveexact(), just in case.

Removed WcsError::new ** breaking change *