This commit adds an option to create an on-disk key-value store that contains all the data from PackageInfo of each indexed package in a queriable form. The store can be used by haskell-code-server to respond to API requests. The main benefit of using the store, compared to deserializing and loading PackageInfo of each package into memory, is reduced memory usage (approximately 7 times for a set of Haskell packages).
The key-value store on disk consists of two files: 'index' and 'values'. 'index' is a small file that contains a map from strings to locations in a 'values' file. 'index' file should be deserialized and loaded into memory. 'values' is a large file that contains serialized Haskell data structures. 'values' file can be either read directly (without deserializing) or memory-mapped.
