Switch cli arg parsing from structopt to clap

[dtolnay]

This eliminates several dependencies (heck, proc-macro-error, proc-macro-error-attr, structopt, structopt-derive, syn-mid, unicode-segmentation) and makes it somewhat more natural to exercise more control over the help output.

Before:

cxxbridge 0.3.4
David Tolnay <[email protected]>
https://github.com/dtolnay/cxx

USAGE:
    cxxbridge <input>.rs              Emit .cc file for bridge to stdout
    cxxbridge <input>.rs --header     Emit .h file for bridge to stdout
    cxxbridge --header                Emit rust/cxx.h header to stdout

FLAGS:
    -h, --help       Print help information
        --header     Emit header with declarations only
    -V, --version    Print version information

OPTIONS:
        --cxx-impl-annotations <cxx-impl-annotations>
            Optional annotation for implementations of C++ function wrappers that may be exposed to Rust. You may for
            example need to provide __declspec(dllexport) or __attribute__((visibility("default"))) if Rust code from
            one shared object or executable depends on these C++ functions in another
    -i, --include <include>...                           Any additional headers to #include

ARGS:
    <input>    Input Rust source file containing #[cxx::bridge]

After:

cxxbridge 0.3.4
David Tolnay <[email protected]>
https://github.com/dtolnay/cxx

USAGE:
    cxxbridge <input>.rs              Emit .cc file for bridge to stdout
    cxxbridge <input>.rs --header     Emit .h file for bridge to stdout
    cxxbridge --header                Emit rust/cxx.h header to stdout

ARGS:
    <input>
            Input Rust source file containing #[cxx::bridge].

OPTIONS:
        --cxx-impl-annotations <annotation>
            Optional annotation for implementations of C++ function wrappers
            that may be exposed to Rust. You may for example need to provide
            __declspec(dllexport) or __attribute__((visibility("default")))
            if Rust code from one shared object or executable depends on
            these C++ functions in another.

    -h, --help
            Print help information.

        --header
            Emit header with declarations only.

    -i, --include <include>...
            Any additional headers to #include. The cxxbridge tool does not
            parse or even require the given paths to exist; they simply go
            into the generated C++ code as #include lines.

    -V, --version
            Print version information.