Merge implementations of read_exact functions/methods

The old functions/methods still exist, so that they continue to raise
the same exceptions as before (which are different depending on
context), but they now use the same implementation internally.

rsrcfork/_io_utils.py

@@ -0,0 +1,18 @@
+"""A collection of utility functions and classes related to IO streams. For internal use only."""
+
+import typing
+
+
+def read_exact(stream: typing.BinaryIO, byte_count: int) -> bytes:
+	"""Read byte_count bytes from the stream and raise an exception if too few bytes are read (i. e. if EOF was hit prematurely).
+	
+	:param stream: The stream to read from.
+	:param byte_count: The number of bytes to read.
+	:return: The read data, which is exactly ``byte_count`` bytes long.
+	:raise EOFError: If not enough data could be read from the stream.
+	"""
+	
+	data = stream.read(byte_count)
+	if len(data) != byte_count:
+		raise EOFError(f"Attempted to read {byte_count} bytes of data, but only got {len(data)} bytes")
+	return data