Adding Ruleset statistics at compile time

[Derekt2]

This adds YARA statistics to this python library. Addresses: https://github.com/VirusTotal/yara-python/issues/224

After a ruleset is compiled, a new method may be called .stats() as follows:

import yara

rule = yara.compile(
    source=(
        'rule foo: bar {strings: $a = "dsfsd" condition: $a} rule foo1: bar {strings:'
        ' $a = "dsfsd" condition: $a}'
    )
)
matches = rule.match(data="abcdefgjiklmnoprstuvwxyz")
print(rule.stats())

which produces output: {'num_rules': 2, 'num_strings': 2, 'ac_matches': 2, 'ac_tables_size': 512} exactly as if you used the YARA command line flag --print-stats

I verified the code works locally, but apologies if I missed styling or best practices as this is my first stab at C.

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