Read and write type nodes through the accessors in Ctype

Ctype, the core of unification, instantiation, generalization and the
occur checks, now reads type nodes only through `get_desc`, `get_level`
and `get_id`, and makes its unlogged writes (copy marks, level marks,
temporary links, writes after `log_type`) through `Transient_expr`. Its
local `get_level env p`, the binding level of a path, is renamed
`get_path_level` so that it does not shadow `Types.get_level`.

Review: Does the conversion change evaluation order, sharing or
allocation? No. Writes still evaluate the new descriptor before storing
it, `filter_visited` returns the same list cell, and the already-copied
variant case in `copy` matches with a guard instead of building an option.

Signed-off-by: Jono Prest <jjprest@gmail.com>

Author: JonoPrest

changes/type-expr-accessors.internal.md

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+- Read and update type nodes in `Btype` and `Ctype` through accessors, a first step towards an abstract `type_expr`.