v0.7.0 - add ability to support multiple Cores; rename EpoxyProm{Avr,Esp} to EpoxyEeprom{Avr,Esp}; add -Wextra compiler fag

• 0.7 (2021-04-28)
  • Add libraries/EpoxyMockDigitalWriteFast, a simple mock library for
    digitalWriteFast (https://github.com/NicksonYap/digitalWriteFast) to
    allow code using these functions (e.g. digitalWriteFast()) to compile
    under EpoxyDuino.
  • Rename EpoxyPromAvr to EpoxyEepromAvr, and EpoxyPromEsp to
    EpoxyEepromEsp, for consistency and better self-description.
  • Add EPOXY_CORE and EPOXY_CORE_PATH variables to allow alternate
    Arduino Core files to be specified. Move current Core files from top-level
    into ./cores/epoxy/ subdirectory.
  • Add memcpy_P() and vsnprintf_P() (Thanks pmp-p@, #28).
  • Add EXTRA_CXXFLAGS to allow additional c++ flags to be specified.
  • Add -Wextra (in addition to -Wall) because some Arduino platforms
    enable both when "warnings" are enabled, so it's convenient to catch those
    warnings on the desktop when using EpoxyDuino.