* transform: size a worker region for the branches of a dense `match`
A worker of the parallel transform appends into a fixed share of the AST,
split by the cost of its functions, which is their node count. Lowering a
`match` appends a condition, a block and a conditional per branch: 7 child
links for a branch of 4 nodes in a dense `return match`, against the 6.7
that the pool gives 4 nodes. A function that is one such `match` of 5000
branches, in a region of its own, ran out:
V panic: array.ensure_cap: array with the flag `.nogrow` cannot grow
in size
Count three more per branch in the cost of the function.
* transform: preserve IError storage for imported pointer casts
