Add RULES for realToFrac from Int.

Add RULES for realToFrac from Int.

    {-# RULES
    "realToFrac/Int->Double"    realToFrac   = int2Double
    "realToFrac/Int->Float"     realToFrac   = int2Float
      #-}

Note that this only matters for realToFrac. If you've been using
fromIntegral to promote Int to Doubles, things should be fine as they are.

The following program, using stream fusion to eliminate arrays:

    import Data.Array.Vector

    n = 40000000

    main = do
          let c = replicateU n (2::Double)
              a = mapU realToFrac (enumFromToU 0 (n-1) ) :: UArr Double
          print (sumU (zipWithU (*) c a))

Yields this loop body without the RULE:

    case $wtoRational sc_sY4 of ww_aM7 { (# ww1_aM9, ww2_aMa #) ->
    case $wfromRat ww1_aM9 ww2_aMa of tpl_X1P { D# ipv_sW3 ->
    Main.$s$wfold
      (+# sc_sY4 1)
      (+# wild_X1i 1)
      (+## sc2_sY6 (*## 2.0 ipv_sW3))

And with the rule:

   Main.$s$wfold
      (+# sc_sXT 1)
      (+# wild_X1h 1)
      (+## sc2_sXV (*## 2.0 (int2Double# sc_sXT)))

The running time of the program goes from 120 seconds to 0.198 seconds
with the native backend, and 0.143 seconds with the C backend.

And just so I don't forget, here's the difference in resulting
assembly (x86_64), between the native code generator, and the
C backend.

-fasm

    Main_zdszdwfold_info:
      movq %rdi,%rax
      cmpq $40000000,%rax
      jne .LcZK
      jmp *(%rbp)
    .LcZK:
      cmpq $39999999,%rsi
      jg .LcZN
      cvtsi2sdq %rsi,%xmm0
      mulsd .LnZP(%rip),%xmm0
      movsd %xmm5,%xmm7
      addsd %xmm0,%xmm7
      incq %rax
      incq %rsi
      movq %rax,%rdi
      movsd %xmm7,%xmm5
      jmp Main_zdszdwfold_info

With the C backend we get the even better assembly, (-fvia-C -optc-O3)

    Main_zdszdwfold_info:
        cmpq    $40000000, %rdi
        je  .L9
    .L5:
        cmpq    $39999999, %rsi
        jg  .L9
        cvtsi2sdq   %rsi, %xmm0
        leaq    1(%rdi), %rdi
        addq    $1, %rsi
        addsd   %xmm0, %xmm0
        addsd   %xmm0, %xmm5
        jmp Main_zdszdwfold_info
    .L9:
        jmp *(%rbp)

So might make a useful test once the native codegen project starts up.