[util] Register constructors inside isolates

Constructors are stored as static class members and ctor created in a
different execution environment overwrites one previously created in
different environment, resulting in different weird effects. This
patch changes the way how environment-wide data is stored: it is
stored now as the isolate data member (struct IsolateContext).

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>