Fix verbose print of instrs that omit low imm bits

Modify `unscatter` so it works for instructions that do not encode
the lowest bits of the immediate value provided to them.
This issue has been hidden because many instructions have been encoded
using the lower immediate bits rather than upper immediate bits
(e.g. `imm[19:0]` rather than `imm[31:12]` for `lui`),
and others (which do omit lower immediate bits) are not included
in disassembly output (e.g. `c_lui` with empty `rv_c_disass`).

See also:
#41

src/InstrCodec.hs

@@ -122,9 +122,11 @@ subst :: Mapping -> BitList -> BitList
 subst m bs = unscatter [(bi, bs !! si) | (bi, si) <- m]
 
 -- Join a scattered bit-string, complain if gaps or overlapping
-unscatter :: [(Int, a)] -> [a]
-unscatter = join 0
+unscatter :: [(Int, Bool)] -> BitList
+unscatter m = zeros ++ join lsb m
   where
+   lsb = minimum [j | (j, _) <- m]
+   zeros = take lsb $ repeat False
    join _ [] = []
    join i m =
      case [x | (j, x) <- m, i == j] of