Extra 4MB of PSRAM on esp32 under Arduino

Some ESP32 modules come equipped with PSRAM, and some even have 8 MB of it.

Unfortunately, only 4 MB of PSRAM is available via standard PSRAM API.

The "high" 4 MB area requires the use of the espressif's HIMEMAPI which (as of Arduino Core 1.0.6) does not work with Arduino Core.

This library breaks that spell and makes the 4 MB of himem area available as a class derived from Stream so you can easily use those megabytes as file storage or as a memory array.

API definition is fairly straightforward and all memory remapping is happening behind the scenes.

Because of the memory remapping the access is rather slow (especially random), so consider this to be a 4 MB fast HDD for ESP32 :)