Remove constructors taking implicit string sizes

Remove constructors taking implicit string sizes

Constructors taking explicit sizes got a default -1 size argument that
triggers length calculation from nul-terminated strings.

This imposes a slight change in behavior: negative size arguments would
previously be ignored and generate an empty string whereas with this
patch we expect to see a nul-terminated string.

On the other hand, keeping the previous behavior could effectively hide
errors in user code and I can't find a good reason to support it.

Documentation for the constructors was updated and made more consistent
between the classes.

Change-Id: I738ac3298cffe3221c8a56e85ba2102623e7b67d
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>