Removed unneeded line from bs_stream_empty
Reset fork status for tests/bs.c
Added unit tests for bs_stream
Correction to tests to stop compiler warnings
Used the C library for memory operations in bs.c

Modified the following methods:
 - bs_zero
 - bs_load
 - bs_save

These all used to have for-loops over the stream's data. These have now been
replaced with calls to the standard library - memcpy and memset. This means:
 - we get to use the heavily-optimised library calls
 - we get to delete some code
Added some more assertions to BS_ASSERT_VALID macro