Support case insenstive strings in yara rules

[msm-code]

Right now, we just ignore strings with the nocase flag:

rule CaseInsensitiveTextExample
{
    strings:
        $text_string = "foobar" nocase

    condition:
        $text_string
}

Supporting them correctly is... harder than it looks like. This can match:

foobar
foobaR
foobAr
foobAR
fooBar
... and 59 strings more

and ursadb query language is not expressive enough to support this.

We can't hack around this by chopping the query in the backend to something like:

( "foo" AND (
    "oob" AND (
        "oba" AND (
            ...
        ) OR
        "obA" AND (

        )
    ) OR
    "ooB" AND (
        "oBa" AND (
            ...
        ) OR
        "oBA" AND (
            ...
        )
    )
) OR "foO AND (
    ...
) OR "fOo" AND (
    ...
) OR "fOO" AND (
    ...
) ...

Because of exponential growth.

OTOH I feel like like this can solved with a C++ method (needs investigation). In this case we need to introduce nocase strings to ursadb.

Needs investigation (if this results in too many false positives, we may as well give up).

[msm-code]

Backend work is done, now then only thing that left is transforming "asdf" nocase into { (61 | 41) (53 | 73) (44 | 64) (66 | 46)}