all: remove theory import path references

In Isabelle2020, when isabelle jedit is started without a session
context, e.g. `isabelle jedit -l ASpec`, theory imports with path
references cause the isabelle process to hang.

Since sessions now declare directories, Isabelle can find those files
without path reference and we therefore remove all such path references
from import statements. With this, `jedit` and `build` should work with
and without explicit session context as before.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

licenses: convert license tags to SPDX

lib: Allow additional rules for `datatype_schem`.

Previously, the method `datatype_schem` used a specific list of
hard-coded rules to "fix" datatypes in schematics. This adds an
attribute so users can add new datatype "lenses"/"accessors" as needed.

Isabelle2018: new "op x" syntax; now is "(x)"

(result of "isabelle update_op -m <dir>")

lib: Datatype_Schematic and WPFix.

Add two new tactics/methods which can fix common painful problems with
schematic variables.

Method datatype_schem improves unification outcomes, by making judicious use of
selectors like fst/snd/the/hd to bring variables into scope, and also using a
wrapper to avoid singleton constants like True being captured needlessly by
unification.

Method wpfix uses strengthen machinery to instantiate rogue postcondition
schematics to True and to split precondition schematics that are shared across
different sites.