diskann vector binary format

[sivukhin]

Context

First branch in the series for DiskANN implementation. This PR introduce few utility classes and accessors for DiskANN node block in binary format.

DiskANN node block v1 format has following structure:

[u64       nodeId] 
[u16       nEdges] 
[vector      node] 
[vector      edge]               * nEdges 
[vector   padding]               * (nMaxEdges - nEdges) 
[[u64 legacyField] [u64 edgeId]] * nEdges

Where nMaxEdges value determined from node block size and node and edge vector sizes. Note, that node vector size can be different from edge vector size (for example, due to edge vector compression)

legacyField were added by Pekka but new implementations of DiskANN don't use it. But I preserved it for compatibility reasons.

Changes

• Add vectordiskann.c and vectorIndexInt.h files (we will continue to add more source code to these files)
• Add BlobSpot helper class which aims to simplify reusal of opened blobs and favour sqlite3_blob_reopen API when possible in order to avoid unnecessary close/open calls
• Add a bunch of nodeBin* methods which operates over raw binary representation and change it accordingly to DiskANN algo needs

Testing

• This branch were extracted from vector branch and it has some tests which validates basic scenarios
• Also, test_libsql_diskann.c were added with some basic tests for added functionality
  • You can compile it with cc test_libsql_diskann.c -I ../ -L ../.libs -llibsql -o test_libsql_diskann
  • You can run it like this:

    $> LD_LIBRARY_PATH=../.libs ./test_libsql_diskann
    debug blob content for root=2 (buffer size=74)
    nEdges=3, nMaxEdges=3, vector=12.340000 
    to=0, vector=2.200000 
    to=1, vector=1.100000 
    to=2, vector=3.300000 
    debug blob content for root=2 (buffer size=74)
    nEdges=2, nMaxEdges=3, vector=12.340000 
    to=0, vector=2.200000 
    to=1, vector=1.100000 
    debug blob content for root=2 (buffer size=74)
    nEdges=3, nMaxEdges=3, vector=12.340000 
    to=0, vector=2.200000 
    to=1, vector=3.300000 
    to=2, vector=1.100000 
    debug blob content for root=2 (buffer size=74)
    nEdges=2, nMaxEdges=3, vector=12.340000 
    to=0, vector=3.300000 
    to=1, vector=1.100000