Investigate moving most of native IModelElementContext.nativeContext.cloneElement into JavaScript

[MichaelBelousov]

The native components of the transformer (chiefly the IModelElementCloneContext's nativeContext) are potentially somewhat redundant to changes in the exporter's javascript side now.

• storing mappings (e.g. element remap table) in native code can be slow compared to using a javascript map.
• harder for our team to maintain.
• harder for C++ and JS to share core code, C++ should only be used where necessary and not tied to the transformer state as it is now

We should explore moving some more stuff into JavaScript.

[MichaelBelousov]

https://github.com/iTwin/imodel-native/tree/geomremap-from-sql-no-clone-rebase-on-memleak-fix

[MichaelBelousov]

things we need:

• CompactRemapTable
• remapping precalculation
• independently cloning geometry blobs
• JavaScript remapping of ids (the asynchronous change should be removed)
• put it all together... these ended up being independently developed on separate branches for different performance tests, so they do need to be stitched together