Introduce a standard thunk for allocating strings

Currently for a top-level closure in the form

    hey = unpackCString# x

we generate code like this:

    Main.hey_entry() //  [R1]
             { info_tbls: [(c2T4,
                            label: Main.hey_info
                            rep: HeapRep static { Thunk }
                            srt: Nothing)]
               stack_info: arg_space: 8 updfr_space: Just 8
             }
         {offset
           c2T4: // global
               _rqm::P64 = R1;
               if ((Sp + 8) - 24 < SpLim) (likely: False) goto c2T5; else goto c2T6;
           c2T5: // global
               R1 = _rqm::P64;
               call (stg_gc_enter_1)(R1) args: 8, res: 0, upd: 8;
           c2T6: // global
               (_c2T1::I64) = call "ccall" arg hints:  [PtrHint,
                                                        PtrHint]  result hints:  [PtrHint] newCAF(BaseReg, _rqm::P64);
               if (_c2T1::I64 == 0) goto c2T3; else goto c2T2;
           c2T3: // global
               call (I64[_rqm::P64])() args: 8, res: 0, upd: 8;
           c2T2: // global
               I64[Sp - 16] = stg_bh_upd_frame_info;
               I64[Sp - 8] = _c2T1::I64;
               R2 = hey1_r2Gg_bytes;
               Sp = Sp - 16;
               call GHC.CString.unpackCString#_info(R2) args: 24, res: 0, upd: 24;
         }
     }

This code is generated for every string literal. Only difference between
top-level closures like this is the argument for the bytes of the string
(hey1_r2Gg_bytes in the code above).

With this patch we introduce a standard thunk in the RTS, called
stg_MK_STRING_info, that does what `unpackCString# x` does, except it
gets the bytes address from the payload. Using this, for the closure
above, we generate this:

    Main.hey_closure" {
        Main.hey_closure:
            const stg_MK_STRING_info;
            const 0; // padding for indirectee
            const 0; // static link
            const 0; // saved info
            const hey1_r1Gg_bytes; // the payload
    }

This is much smaller in code.

Metric Decrease:
    T10421
    T11195
    T12150
    T12425
    T16577
    T18282
    T18698a
    T18698b

Co-Authored By: Ben Gamari <ben@well-typed.com>