feat: add bloom_filter lib files using xxhash64 internally

DEV.md

@@ -22,3 +22,69 @@ This double hashed func almost behaves like a random number generator.
 ## std::hash
 
 you make a hasher object and them immediately call it
+
+## Designing the api
+
+Now that I have a basic version ready I need to design some sort
+of an api going forward - for the header. For now I'm sticking
+to the traditional impl.
+
+There was a google's impl
+https://github.com/google/guava/blob/master/guava/src/com/google/common/hash/BloomFilter.java
+
+and also another cpp lib that's around 10y old
+https://www.partow.net/programming/bloomfilter/index.html
+
+Ok going step by step, first the hash function
+
+### Hash func
+
+The guava one uses MurmurHash3 ( first I've heard of it )
+There are two sorts of hashes - cryptographic and non crypto
+
+The diff is explained here: https://security.stackexchange.com/questions/11839/what-is-the-difference-between-a-hash-function-and-a-cryptographic-hash-function
+
+The point being - cryptographic ones ensure security against some
+adversary and guarantee some stuff like
+1. preimage resistance - given hash find message
+2. second preimage resitance, given just x and h(x), find another x'
+with same hash
+3. collision resistance - free to choose any pair but need to match
+hashes
+
+When you loosen these constraints - faster hashes are possible.
+
+In any case - I googled and seems like xxhash is the fastest now
+so I'll use that.
+
+From there I downloaded the simplest impl
+https://create.stephan-brumme.com/xxhash/ the v2 version for 64.
+It's a single header so that should be good enough.
+
+### Back to actual api?
+
+I liked Google's api for it - especially the naming and all.
+I won't b supporting any set operations at all.
+
+mightContain(x)
+expectedFalsePositiveProbab
+put
+approxElemCount() - estimates from fraction of set bits
+
+Constructors - there should be these atleast
+(expectedPuts, numHashes, falsePositiveityRate)
+(bitSize, numHashes)
+
+### The headers
+
+using vector<bool> as it's bit packed internally and allows for
+runtime size declaration
+
+I ended up writing the entire impl in the header file and then split it.
+Earlier I was going for a templated thing but that needed a lot of additional handling
+due to how something with internal pointers would manage data ( say std::string ).
+Now it's similar to xxhash where you pass in a void pointer and a length to it.
+
+## Fixing clangd
+
+Post the split, I needed to fix clangd as it did not