equivalence rules for structures without zero divisors

removed obsolete global-only options, which did not work out anyway (due to complexity of local_theory sandwich);

optional proof context for unify operations, for the sake of proper local options;

clarified name of Type.unified, to emphasize its connection to the "unify" family; tuned low-level operation;

proper Envir.norm_type for result of Type.raw_unifys;

avoid slow metis proof;

proper Envir.norm_type for result of Unify.unifiers (amending 479832ff2d29 from 20 years ago);

tuned;

updated some sledgehammer proofs -- much faster;

updated sledgehammer proof after breakdown of metis (exception Type.TUNIFY);

recovered type matching, which was broken in 8a765db7e0f8 (see also 8a765db7e0f8, 2db1d3d2ed54); NB: "match" operates on direct substitution without variable chasing, in contrast to "unify" (and Unify.matches!) which work on cascaded env;

more direct type equality;

reverted commit accident from 994fe0ba8335

less space-wasting serialization setup: highest cell of array has been unused so far

tuned outline;

tuned syntax -- separate tokens;

eliminated pointless check -- command definitions are subject to theory context;

merged

prefer externally provided keywords -- Command.read_thy may degenerate to bootstrap_thy in case of errors; tuned message;

tuned markup;

tuned;

clarified keyword categories; tuned;

tuned signature;

tuned;

plain value Keywords.keywords, which might be used outside theory for bootstrap purposes; plain value Outer_Syntax within theory: parsing requires current theory context; clarified name of Keyword.is_literal according to its semantics; eliminated pointless type Keyword.T; simplified @{command_spec}; clarified bootstrap keywords and syntax: take it as basis instead of side-branch;

proper import for command 'permanent_interpretation';

proper import of Main: BNF_Least_Fixpoint does not "contain pretty much everything", especially it lacks the 'value' command, which is defined *after* theory List;

more accurate keywords;