Rust 1.51.0 warning: panic message is not a string literal

[easbar]

Using Rust 1.51.0 I get:

warning: panic message is not a string literal
   --> src/preparation_graph.rs:150:13
    |
150 | /             format!(
151 | |                 "invalid node id {}, must be in [0, {}[",
152 | |                 node, self.num_nodes
153 | |             )
    | |_____________^
    |
    = note: `#[warn(non_fmt_panic)]` on by default
    = note: this is no longer accepted in Rust 2021
    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

[easbar]

Fixed via #28