Log Message:
gnuradio-core: update to 3.8.1.0

Here is a very high level list of changes:

## Changed
### Project Scope
- C++11
- merged the wholeness of the `next` branch
- Dependency version bumps: CMake, GCC, MSVC, Swig, Boost
- New dependencies: MPIR/GMP, Qt5, gsm, codec2
- Removed dependencies: libusb, Qt4, CppUnit
- Python: Python 2 & Python 3 compatible. 3.8 will be the last Py2k-compatible
  release series
- gengen was replaced by templates (if you don't know gengen, don't do any
  research; save yourself that sorrow)
- Modern CMake (as far as feasible at this point)
- VOLK version updated to v2.0.0
- .clang-format file now dictates coding style
- clang-format'ed the whole tree.
- installed CMake files now tell about configuration

### gnuradio-runtime
- reworked fractional tag time handling, especially in the context of resamplers

### GRC
- C++ generation as option
- YAML instead of XML
- removed `blks2`
- much better canvas tooling
- consistent gobject usage
- ROUNDED ARROWS

### gr-qtgui
- moving from Qt4 to Qt5

### gr-utils
- `gr_modtool` now vastly improved

### gr-vocoder
- improved versatility
- removed in-tree libgsm, libcodec2, use system-wide libs

## Removed
### Project Scope
- Modules `gr-comedi`, `gr-fcd` and `gr-wxgui` are gone
### gr-comedi
- nobody could remember who used this, or for what. It has seen 0 active code
  contributions in the 3.7 lifecycle
### gr-digital
- python-based `packet_encoder` and related tools: Bugs that were sporadic and
  never fixed, so after long deprecation, we're removing it
### gr-fcd
- since it's currently untestable by the CI, it's being removed, as there was no
  code contributions. Generally, we strive to include all batteries with GNU
  Radio. Re-integration within a more general SDR interface would be desirable.
### gr-utils
- removed PyQwt (dead) based tools
### gr-wxgui
- Unmaintained, breaks on increasingly many systems, always was slower than
  Qtgui. We've been starting to tell people to migrate to Qt since at
  least 2015. Now, we're finally removing it.