Add ExecutionContext; converted Contexts to a unified SALVEContext.

Migrated backup globals into exec context, Stream and Exec contexts are held
in a SALVEContext. Backup globals are no longer expanded into blocks, but put
directly into actions via the execution context. All actions carry a SALVEContext
which includes the relevant ExecutionContext.
Error changes: rather than just printing the StreamContext, print the entire
SALVEContext. This just means printing the phase as well as StreamContext.
Included is normalizing paths in errors.
Execution context changes between phases as a SALVE execution runs.
Unfortunately, some steps are muddled at present -- like verification
and execution of actions.
Added context unit tests.

Progress towards test coverage. Includes some running repairs to other
unit tests.
Merge branch 'feature/execution-context' into dev