Add libdwfl based implementation

[kanje]

This commit adds an implementation based on libdwfl from elfutils.

Implements #176

[kanje]

@apolukhin, please have a look at this draft. Note, I was using CMake and b2 support is still missing. I would appreciate some help here as I know nothing about how to write Jamfiles.

[apolukhin]

Tests are missing. You will need to write some b2 scripts: follow this sample https://github.com/boostorg/stacktrace/blob/344f6bae1929c87fa62a601ea53433d2b79ee0c0/build/Jamfile.v2#L80-L93 and add code to search for libdwfl https://github.com/boostorg/stacktrace/blob/344f6bae1929c87fa62a601ea53433d2b79ee0c0/build/Jamfile.v2#L34-L40

After that adjust https://github.com/boostorg/stacktrace/blob/develop/test/Jamfile.v2 by adding each of the tests with libdwfl