Improve tvf! macro for negative numerical values

[oschijns]

Allow defining the sign of a numerical value without wrapping into into extra parenthesis.
from

  let buffer = tvf!(SimpleStringTvf {
      10 => (-10),
      11 => (-10.25),
      12 => (-2) as Signed,
      13 => (+2.125) as Float,
  });

to

  let buffer = tvf!(SimpleStringTvf {
      10 => -10,
      11 => -10.25,
      12 => -2 as Signed,
      13 => +2.125 as Float,
  });