style: Remove needless borrow

[caspermeijn]

This solves clippy warnings like this:

error: this expression creates a reference which is immediately dereferenced by the compiler
   --> src/common.rs:447:34
    |
447 |             unquote_block_string(&block),
    |                                  ^^^^^^ help: change this to: `block`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_borrow)]`