Add ability to customize max bytes() size

[s-hadinger]

Previously the max size for bytes() object was 32KB. For ESP32 this is reasonable, but too small when attached PSRAM of 4/8 MB. It is now possible to change the compile time default BE_BYTES_MAX_SIZE and to dynamically change this size with vm->bytesmaxsize. This makes it possible to dynamically detect the presence of PSRAM and adjust max size accordingly.