Partially implement annotations

[dahlia]

Any declarations should be possible to have zero or more annotations. The syntax would look like the following:

[http-get: "/and/def/:arg"]

An annotation have to take an argument which is a string.

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To be used for service keyword and RPC server/client generator, we need to implement annotations for very minimum range of declarations:

• Service
• Service method

The complete set of annotations could be continued later in #40.