Remove unnecessary mut from variable

[cjpearce]

Just removing an unnecessary mut from the dist_if_substitute variable, as it's common to avoid mut in rust where possible. This has a surprisingly large effect on performance:

before:

go: 1.768400
javascript: 3.54
rust: 2.069717745

after:

go: 1.757984
javascript: 3.347
rust: 1.415786823

Both tests were run on a Dell XPS.