It has the same role as the `cnkBinding` node in the CGIR, that is, to
group a field name and value expression in an object construction
expression together.

Previously, the `mnkField` and expression nodes loosely followed each
other, making `mnkObjConstr` subtrees heterogenous, which complicates
some traversal (or at least it will, once more traversal is tree-
length-based).

`mnkPathNamed`, `mnkPathVariant`, `mnkPathPosition`, and `mnkName` are
all going to change shape. For better forward compatibility, dedicated
construction template for them are added and used.

The `ekNone` effect kind is introduced for being able to signal "no
effect" when constructing a `mnkName` tree.

Instead of requiring delimiter nodes, subtree root nodes now store a
their number of subnodes. Multiple subtree root nodes previously stored
extra information, which is now no longer possible.

Most tree traversal routines are updated accordingly, and those
specific to `mnkEnd` nodes are removed.

Since it's obsolete now, the `GeneralEffect` enum type is removed.

* skipping end nodes is unnecessary and wrong now
* instead of iterating until an `mnkEnd` node is found, a for loop
  using the node count suffices

A `MirBuffer` instance keeps the track of the number of the subnodes in
the currently constructed tree, updating the `len` field in the root
node on subtree completion.

The value is saved on the stack when starting a nested subtree. For
manual subtree construction (i.e., not using the `subTree` template),
the `start` and `finish` procedures are provided.

Since the builder manages updating the length field, manually providing
the number of nodes is unnecessary (`mirgen` is updated accordingly).

Subtree nodes no longer being able to store non-length information
requires the information to be stored elsewhere. The `mnkImmediate`
node is added, and used as an ad-hoc way of storing information in
the tree.

* call trees store whether the call modifies some global state in
  an immediate value in the first slot
* `mnkName` trees store the tag as an immediate value in the first
  subnode -- `mnkTag` nodes are removed
* the named-field-access trees store the field position in an
  `mnkField` node in the second slot
* `mnkPathPos` stores the position as an immediate value in the
  second slot

The tree construction throught the MIR processing is updated
accordingly.

It replaces the now-unavailable `findEnd` + `previous` combination.

* inspection of call, path, and `mnkName` is adjusted to work with the
  new tree shapes
* dedicated query procedures are used instead of manual indexing where
  sensible

There's no more end node that needs to be patched.

Manually constructing a `mnkCall`/`mnkCheckedCall` sub-tree is prone to
mistakes, and it also complicates future changes to the syntax (should
any be made). `mirconstr` now provides the `rawBuildCall` template for
building call trees where full control over the makeup is needed.

Co-authored-by: Saem Ghani <[email protected]>