* [Runtime] Enable the -Wshorten-64-to-32 flag and fix all warnings.
We want to enable the -Wconversion but that will raise too many warning
for a single commit. We are enabiling one by one the flags included in
-Wconversion so that we have smaller diffs.
-Wshorten-64-to-32 adds warnings when there is a implicit conversion that
loses integer precision. We are moving all the 32 to 64 conversions to
use 64. Expecially since most of the code changed is related with sizes,
legths and params counts that are never going to be negative.
Co-Authored-By: Rolf Bjarne Kvinge <rolf@xamarin.com>