Merge branch 'feature/kernel/diff-ast' into 'master'

Makefile

Makefile.generating

configure.in

devel_tools/docker/Makefile

headers/header_spec.txt

nix/default.nix

reference-configuration.md

share/Makefile.config.in

src/kernel_services/ast_queries/ast_diff.ml

src/kernel_services/ast_queries/ast_diff.mli

src/kernel_services/ast_queries/cil_datatype.ml

src/kernel_services/plugin_entry_points/kernel.ml

src/kernel_services/plugin_entry_points/kernel.mli

tests/syntax/ast_diff_1.i

tests/syntax/ast_diff_1.ml

tests/syntax/ast_diff_2.i

tests/syntax/oracle/ast_diff_1.res.oracle