[LyX/master] Reduce the amount of needed boost headers

[Pavel Sanda]

On Sat, Dec 19, 2020 at 11:08:40PM +0200, Yuriy Skalko wrote:
> Yes, boost's `assert` is not encapsulated, but `crc` and `any` are.
> 
> I've moved all into `lyx` namespace. Really separate namespace for only 3
> names is not needed.

BTW after all the boost related changes, what are the dependencies needed for people using external boost?
E.g. current 2.3.x debian build dependencies are libboost-filesystem-dev, libboost-iostreams-dev, libboost-regex-dev, libboost-signals-dev, libboost-test-dev.
Can some of these be removed now?

Pavel