feat: Prefix Bloom Filter

[khlopkov]

This PR supposed to resolve #97

Please, let me know if I missed something.

[marvin-j97]

I don't think this is necessary.
If a key is [1, 2, 3] and we extract the first 4 bytes, we would just take the entire string, so:

let len = Math.min(key.len(), 4);
return key.slice(0, len);

[khlopkov]

Let me better explain why is this needed in more practical example:

Imagine we have a database where keys are constructed using the following template: {datatype}#{id} and a prefix extractor that would extract {datatype}# prefix from a key to optimize scans of all rows of datatype#. Then, in table were populated following rows table1#a, table1#b, table2#a. The keys for bloom filter would be table1#, table2# in this case. Then, if we use prefix scan by prefixes table1# or table2# everything will work correctly. But if scan is performed with table prefix without in_domain the full prefix will be extracted and will try to check bloom filter by table key, which may return false negative. in_domain in this case will help to prevent such cases and will use Bloom Filter only in case when prefix is in domain. In this example everything starting with {datatype}# will be considered as in_domain and use Bloom Filter, other prefixes will be not in_domain and will just use normal prefix search ignoring Bloom Filter

[marvin-j97]

Makes sense.

However, for the prefix extractor to support structured prefixes, the trait needs to return an Iterator over extracted prefixed (see #97 and fjall-rs/fjall#116). At that point, the trait could just return std::iter::empty, which would signal that the key is "not in domain".

For example we may extract prefixes, like:

eu#germany#berlin

-> extract "eu#" as prefix
-> also extract "eu#germany#"

Now prefix searches over both continent and [continent + country] can be filtered by the prefix filter.