Only inline empty functions

The one-true-source-of-visionary-coding-style™ has stated that inline
functions are _not_ visionary style.

https://gerrit.bioai.eu/c/hxcomm/+/10315/38/include/hxcomm/common/stream.h#64

We have to adjust the coding style accordingly.

Change-Id: Ie0602c3eebf99faffaf5ec508b294bf76d4af2e9