Since 8baf4a69 we lost information about
an overflowed workspace by calling std.log(), which resulted from use of
the workspace for construction of a contiguous string from the argument
constituents.

Since then, we have changed the interface to STRANDS, but this issue
remained.

We now solve the case for real by pushing the string concatenation down
to VSL: New versions of VSL and VSLb (coded by example of VSLv() and
VSLbt()) take a strands argument and create a log record without
additional copy overhead.

These solve #3194 for std.log(), make logging more efficient and, in
particular, allow use of std.log() in low workspace conditions (because
they do not require any).

Also improve test coverage for std.log()

Ref #3194