LibCore: Introduce BigEndianInputBitStream

BigEndianInputBitStream is the Core::Stream API's bitwise input stream
for big endian input data. The functionality and bitwise read API is
almost unchanged from AK::BitStream, except that this bit stream only
supports big endian operations.

As the behavior for mixing big endian and little endian reads on
AK::BitStream is unknown (and untested), it was never done anyways. So
this was a good opportunity to split up big endian and little endian
reading.

Another API improvement from AK::BitStream is the ability to specify
the return type of the bit read function. Always needing to static_cast
the result of BitStream::read_bits_big_endian into the desired type is
adding a lot of avoidable noise to the users (primarily FlacLoader).

AK/BitStream.h

@@ -12,6 +12,7 @@
 
 namespace AK {
 
+// Obsoleted by LibCore/{Big, Little}EndianInputBitStream.
 class InputBitStream final : public InputStream {
 public:
     explicit InputBitStream(InputStream& stream)