Add a sketch for static keys cache

[asvetlov]

The idea is: statically cache the most common HTTP headers.

For example, aiohttp.hdrs can do the following:

CONTENT_LENGTH = istr("Content-Length")
multidict.add_static_cache(str(CONTENT_LENGTH), CONTENT_LENGTH)
multidict.add_static_cache(str(CONTENT_LENGTH).lower(), CONTENT_LENGTH)
multidict.add_static_cache(str(CONTENT_LENGTH).upper(), CONTENT_LENGTH)