Finish 01_ interpreter in Rust.

Very slow in a meaningless comparison between interpreters in
  Rust    in 01_/rs:        1.235real 0.014sys
  Go      in 01_/go:        0.016real 0.010sys
  C       in 01_/c:         0.008real 0.003sys
  Haskell in 01_/hs/interp: 0.004real 0.005sys

running this 01_ code once:
q=d p d0010111000001010.p0b=00110000p b.p1b=00110001p b.p_=_.d=011100010011110101100100001000000111000000100000011001000011000000110000001100010011000000110001001100010011000100110000001100000011000000110000001100000011000100110000001100010011000000101110011100000011000001100010001111010011000000110000001100010011000100110000001100000011000000110000011100000010000001100010001011100111000000110001011000100011110100110000001100000011000100110001001100000011000000110000001100010111000000100000011000100010111001110000010111110011110101011111001011100110010000111101.