Work around syntect regression

[dtolnay]

#123 is needed after all, since syntect 4.7.1 is also broken. See trishume/syntect#402.

$ cargo install cargo-expand

$ cargo expand
   Compiling proc-macro2 v1.0.36
    Finished dev [unoptimized + debuginfo] target(s) in 0.44s

memory allocation of 7881122326526296064 bytes failed
Aborted (core dumped)