bitvec: Add get/set byte sequences

The new functions bitvec_get_bytes and bitvec_set_bytes copy
byte sequences from bitvecs to uint8_t arrays and vice versa.
While the bytes in the bitvecs do not need to be aligned, the uint8_t
arrays always are. In case the bytes in the bitvec are aligned, the
implementation uses memcpy.

Note that the implementation like the other existing functions assume
MSB first encoding.

[hfreyther: Squash the comment fix into this commit as well]

Sponsored-by: On-Waves ehf
diff --git a/tests/bitvec/bitvec_test.ok b/tests/bitvec/bitvec_test.ok
new file mode 100644
index 0000000..1f329af
--- /dev/null
+++ b/tests/bitvec/bitvec_test.ok
@@ -0,0 +1,2 @@
+=== start test_byte_ops ===
+=== end test_byte_ops ===