style: Replace unnecessary map_or

[caspermeijn]

Fixes clippy lints like this:

warning: this `map_or` is redundant
   --> prost-build/src/ast.rs:104:9
    |
104 | /         chars
105 | |             .next()
106 | |             .map_or(false, |c| c != ' ' || chars.next() == Some(' '))
    | |_____________________________________________________________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_map_or
    = note: `#[warn(clippy::unnecessary_map_or)]` on by default