Bug 1821061 - wasm: Rename to use 'instanceData' consistently. r=jseward

We use many variations of 'global data', 'global area', and more recently
'instance data'. I prefer 'instance data' as 'global' is ambiguous with
wasm globals, which while stored in the instance are not the only thing
stored there. The naming scheme for this originated when globals were
the only thing stored there, so we should now change it.

Depends on D172026

Differential Revision: https://phabricator.services.mozilla.com/D172027