The operation now expects its operands to be `NimNode`s already. A new
instruction (`opcDataToAst`) is introduced for creating the AST
representation of VM data.

While also simplifying the VM a bit, the main reason behind the change is
to not having to provide the full AST of the template call expression,
as doing so is not possible when the code generator no longer operates
on `PNode` AST.

The `nfAllFieldsSet` flag stopped reaching the code generator with the
introduction of the MIR, meaning that the condition always evaluates to
'true'.

The C and JS code-gen orchestrators were passing the AST produced for
the main procedure directly to the code generators. This is no longer
going to work once the code generators don't work with `PNode` anymore,
so `canonicalize` is now used on the AST.

Instead of an integer-literal node, the procedure now accepts the value
directly.

`astgen` is adjusted to produce `CgNode` instead of `PNode`. For this,
multiple `PNode` -> `CgNode` translation procedures had to be
introduced and `canUseView` + `flattenExpr` duplicated and adjusted for
`CgNode`. The general processing logic stays the same.

The module's document comment is also adjusted and an outdated mention
of "sections" (they are called "regions" in the MIR) fixed.

`astgen` as the name doesn't make much sense anymore and is going to be
changed to something more fitting.

`canonicalize` and `generateAST` now return `CgNode` trees. For debug
rendering, a `treeRepr` procedure for `CgNode` is added to the
`cgirutils` module.

The instruction-emission procedure now accept a `TLineInfo` as input
directly, instead of, unnecessarily, requiring a `PNode`. Wrappers that
still use `PNode` are added for convenience.

All three code generators now use the `CgNode` IR. The changes to the
modules are kept minimal in order to make review easier.

As an additional way to keep the amount of changes smaller, the `compat`
module is introduced.

It's obsolete now.

In addition, the module is moved to the `backend` directory.

The module is named `cgirgen` now. `debug.rst` also contained outdated
mentions of `PNode` being the IR the code generators -- this is fixed
too.