[Kernel] add statement attributes (sattr) field; use sattr for loops

src/kernel_internals/typing/cabs2cil.ml

src/kernel_internals/typing/unroll_loops.ml

src/kernel_services/ast_data/cil_types.mli

src/kernel_services/ast_printing/cil_printer.ml

src/kernel_services/ast_printing/printer.ml

src/kernel_services/ast_printing/printer_api.mli

src/kernel_services/ast_queries/cil.ml

src/kernel_services/ast_queries/cil.mli

src/kernel_services/ast_queries/cil_datatype.ml

src/plugins/gui/pretty_source.ml

src/plugins/slicing/printSlice.ml