read receive count & fetch (blocking) a fixed count of bytes

[incanus]

This adds a complementary feature to checking whether the Rx FIFO has bytes, which is getting the actual count of bytes available in it using write_data_bytes_available().

It also adds a method to read, blocking, a specific count of bytes. Unlike get_write_data(), which will return up to a certain count, get_write_data_blocking() will loop on pass until the full byte count is available. Obviously some care should be used to check that the count is available first.

Please let me know if this is in line with the project goals, and thanks for providing this library! It's been very useful.