Add ANN search support for vec0 virtual table (#273)

Add approximate nearest neighbor infrastructure to vec0: shared distance
dispatch (vec0_distance_full), flat index type with parser, NEON-optimized
cosine/Hamming for float32/int8, amalgamation script, and benchmark suite
(benchmarks-ann/) with ground-truth generation and profiling tools. Remove
unused vec_npy_each/vec_static_blobs code, fix missing stdint.h include.

benchmarks/profiling/build-from-npy.sql

@@ -8,10 +8,3 @@ create virtual table vec_items using vec0(
   embedding float[1536]
 );
 
--- 65s (limit 1e5), ~615MB on disk
-insert into vec_items
-  select
-    rowid,
-    vector
-  from vec_npy_each(vec_npy_file('examples/dbpedia-openai/data/vectors.npy'))
-  limit 1e5;