Add simple thread example

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix cygwin compilation error

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix types of copy/move constructors in interval.h
Add missing initialization of numeric_traits<mpfp>::rnd
Add double/mpfp interval tests
Fix compile-error by adding "defined" to interval_def.h