Multiple macros expansions with bindings results in exponential time complexity

Created by: andreypopp

Using code from master branch.

Example macro

macro m {
  rule { $val:expr } => {
    var x = $val;
  }
}

m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11

expands in 1.04s

While

m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11

expands in 5.34s

And after adding just a one more m 11

m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11
m 11

in 10.26s

:(