internal: use dedicated context type for vmbackend

[zerbina]

Summary

Add and use a dedicated type for the contextual state of the VM code-
generation orchestrator. This removes the hard coupling between
vmbackend and vmdef.TCtx and is another preparation for splitting up
the latter.

Details

The new GenCtx type replaces TCtx as the contextual-state type
for the orchestrator. For now, GenCtx still stores an instance of
TCtx, as the latter currently contains the code generator state.

In preparation for splitting up TCtx, usage of its idgen and
functions fields are removed from vmbackend.nim. The ID generator is
directly passed to procedures that need one, and the function table
(functions) is now stored in GenCtx -- prior to serializing the VM
environment, the function table stored with GenCtx's is moved into the
TCtx instance.

With the new dedicated type for the orchestrator, the collected globals
don't have to be stored in TCtx anymore. The collectedGlobals field
is made part of GenCtx and is renamed to just globals.

Misc

• add the base template for OrdinalSeq and use it to reduce the
  noise in the OrdinalSeq routines
• fix the add template for OrdinalSeq

[saem]

good start to the split. :D

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• part of the general TCtx split-up. The short-term goal is to separate the vmgen state from TCtx, which is effectively required for multiple planned CG-IR improvements