Compiler is unable to generate as efficient code for `write!(f, "text")` as it does for `f.write_str("text")`.  This PR checks if the `#[error("text")]` uses a simple string literal without the `{` and `}` characters, and without arguments, and uses `write_str` if so.

Optimize performance for string literals in Display::fmt

    warning: unnecessary hashes around raw string literal
       --> tests/test_display.rs:354:12
        |
    354 |     assert(r#"raw brace left {"#, Error::BraceLeft);
        |            ^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
        = note: `-W clippy::needless-raw-string-hashes` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::needless_raw_string_hashes)]`
    help: remove all the hashes around the string literal
        |
    354 -     assert(r#"raw brace left {"#, Error::BraceLeft);
    354 +     assert(r"raw brace left {", Error::BraceLeft);
        |

    warning: unnecessary hashes around raw string literal
       --> tests/test_display.rs:355:12
        |
    355 |     assert(r#"raw brace left 2 \x7B"#, Error::BraceLeft2);
        |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
    help: remove all the hashes around the string literal
        |
    355 -     assert(r#"raw brace left 2 \x7B"#, Error::BraceLeft2);
    355 +     assert(r"raw brace left 2 \x7B", Error::BraceLeft2);
        |

    warning: unnecessary hashes around raw string literal
       --> tests/test_display.rs:356:12
        |
    356 |     assert(r#"raw brace right }"#, Error::BraceRight);
        |            ^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
    help: remove all the hashes around the string literal
        |
    356 -     assert(r#"raw brace right }"#, Error::BraceRight);
    356 +     assert(r"raw brace right }", Error::BraceRight);
        |

    warning: unnecessary hashes around raw string literal
       --> tests/test_display.rs:357:12
        |
    357 |     assert(r#"raw brace right 2 \x7D"#, Error::BraceRight2);
        |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
    help: remove all the hashes around the string literal
        |
    357 -     assert(r#"raw brace right 2 \x7D"#, Error::BraceRight2);
    357 +     assert(r"raw brace right 2 \x7D", Error::BraceRight2);
        |