[region] pretty print field offsets

ivette/src/dome/renderer/graph/diagram.tsx

ivette/src/frama-c/plugins/region/api/index.ts

ivette/src/frama-c/plugins/region/memory.tsx

src/libraries/utils/json.mli

src/libraries/utils/json.mll

src/plugins/region/fields.ml

src/plugins/region/fields.mli

src/plugins/region/memory.ml

src/plugins/region/memory.mli

src/plugins/region/services.ml

src/plugins/region/tests/region/oracle/blob.res.oracle

src/plugins/region/tests/region/oracle/comp.res.oracle

src/plugins/region/tests/region/oracle/fb_ADD.res.oracle

src/plugins/region/tests/region/oracle/fb_SORT.res.oracle