1.3 - unitduino emulation on Linux and MacOS

• 1.3 (2019-06-15)
  • Remove direct creation of String object, removing last remaining dynamic
    allocation of heap memory from library.
  • Add experimental implementation of unitduino, an Anduino emulation
    layer that allows AUnit tests to compile and run on Linux and MacOS.
  • Fix broken compile of print64.cpp on ESP8266 Arduino Core 2.5.1.
  • Use class instead of struct in the various test() macros when
    creating subclasses for each test. Fixes warnings about mismatch between
    'struct' and 'class' in clang++ when forward declaring the tests in code
    that gives access to private members to tests.