macros: introduce `fluent_messages` macro

Adds a new `fluent_messages` macro which performs compile-time
validation of the compiler's Fluent resources (i.e. that the resources
parse and don't multiply define the same messages) and generates
constants that make using those messages in diagnostics more ergonomic.

For example, given the following invocation of the macro..

```ignore (rust)
fluent_messages! {
    typeck => "./typeck.ftl",
}
```
..where `typeck.ftl` has the following contents..

```fluent
typeck-field-multiply-specified-in-initializer =
    field `{$ident}` specified more than once
    .label = used more than once
    .label-previous-use = first use of `{$ident}`
```
...then the macro parse the Fluent resource, emitting a diagnostic if it
fails to do so, and will generate the following code:

```ignore (rust)
pub static DEFAULT_LOCALE_RESOURCES: &'static [&'static str] = &[
    include_str!("./typeck.ftl"),
];

mod fluent_generated {
    mod typeck {
        pub const field_multiply_specified_in_initializer: DiagnosticMessage =
            DiagnosticMessage::fluent("typeck-field-multiply-specified-in-initializer");
        pub const field_multiply_specified_in_initializer_label_previous_use: DiagnosticMessage =
            DiagnosticMessage::fluent_attr(
                "typeck-field-multiply-specified-in-initializer",
                "previous-use-label"
            );
    }
}
```

When emitting a diagnostic, the generated constants can be used as
follows:

```ignore (rust)
let mut err = sess.struct_span_err(
    span,
    fluent::typeck::field_multiply_specified_in_initializer
);
err.span_default_label(span);
err.span_label(
    previous_use_span,
    fluent::typeck::field_multiply_specified_in_initializer_label_previous_use
);
err.emit();
```

Signed-off-by: David Wood <david.wood@huawei.com>

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