Btree tests: Use std::string.

Masstree string_type has a shallow copy; we want a deep copy.
Actually test Masstree scans.

Requires some code changes. Breaks currently.
Avoid tarded valgrind warnings
Ensure the provision of masstree print().
Make tree print output tolerable
Use new masstree basic_table::destroy().
Masstree has separately-configurable precondtions and invariants.

The fact that we turned on only invariants (and not preconditions)
with CHECK_INVARIANTS=1 was very confusing.
Configure masstree to take big enough keys.
mp_test7: Need to make copies of scanned strings.
Readable keys preferred