Add conditional joins and table views to Pure modules

Module format v5 lets a value cross a join: the verifier records the
stack each branch brings to its target and requires every path to agree
on height, types and owners, with at most 8 such joins ahead. Branches
are now allowed in views, so a view can be a conditional expression.

A table view has one row per item of a list. Its columns are number or
text views evaluated for one row at a time and may read that row (ROW).
Hosts compute only the cells they show through pure_app_rows,
pure_app_cell and pure_app_cell_text. Column names are scoped to their
table. The view limit rises to 64, columns included.

pure-run prints a table view as an array of row objects.

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