The existing class offered several constructors which were mostly
unnecessary. This commit eliminates all existing constructors and
introduces a single new one, taking a `Slice`.
The internal buffer is switched from `std::vector` to `Buffer` to
save a minimum of 8 bytes (plus the buffer slack that is inherent
in `std::vector`) per SHAMapItem instance.
This commit combines a number of cleanups, targeting both the
code structure and the code logic. Large changes include:
- Using more strongly-typed classes for SHAMap nodes, instead of relying
on runtime-time detection of class types. This change saves 16 bytes
of memory per node.
- Improving the interface of SHAMap::addGiveItem and SHAMap::addItem to
avoid the need for passing two bool arguments.
- Documenting the "copy-on-write" semantics that SHAMap uses to
efficiently track changes in individual nodes.
- Removing unused code and simplifying several APIs.
- Improving function naming.
