Add new properties of the Hash form of Fbe.overwrite inside a transaction

[Thayorns]

When every key of the Hash form of Fbe.overwrite was new to the fact, the in-place branch set them one by one on the live fact. Under Fbe.fb the rules checked the fact after each single assignment, so a pair such as repository and where, which the rules accept only together, failed or passed depending on the order of the keys, and a failure left the first key behind in a fact that no longer matched the rules.

The in-place branch now finds the fact by its identifier inside fb.txn and adds the properties there, so the rules see the finished fact once and a failure rolls all of it back, the same way the recreate branch already works. A fact without an identifier is still filled directly. That only happens on a plain Factbase, where there are no rules to break, and Fbe.repeatedly depends on it for its first marker. If the identifier no longer points at any fact, Fbe::Error is raised with the same message the recreate branch uses.

Callers should read the fact again from the factbase after the call, as they already have to after a recreate, because the transaction commits a fresh copy of the fact.

Closes #1242

[Thayorns]

@yegor256 take a look please, happy to clarify anything about the change.