Allow the unused stack to be reset to the check value at any time in
the application, allowing for delta-stack calculations to be done.
Add ESP.resetFreeContStack() class method for general use.
Add in some dumping in the BearSSL_Validation example to show the
usage for those that care.
* Suppressed -Wunused-parameter and -Wunused-function by casting to void unused identifiers.
* Explicit initialization of all fields to suppress -Wmissing-field-initializers.
* Fixed signed/unsigned integer comparison.
* memset initialization of structs.
* More -Wunused-parameter fixes.