Adding a test-suite for bug #3422.
Documenting the change of semantics of the "constructor" tactic.
Documenting the need of the "DECLARE PLUGIN" statement.
More complete printing of Ltac location, akin to the term-dedicated Locate command.
Adding a new "Locate Term" command, distinct from the raw "Locate" command.
The new Term version has essentially the same behaviour as the old "Locate",
while the new raw searches for all types of objects. Also code merging with
the "Locate Ltac".

Fixes bug #3380.
Unifying locate code, also making it more powerful: it is now able to find
any prefix of the given qualid.