feat(bindings/haskell): init haskell binding

[silver-ymz]

This is a demo for haskell binding.

Explanation

I researched 3 ways to complete haskell bindings.

1. Using hs-bindgen
2. According completed C binding
3. Self exporting C-compatible API

The first way is the simplest way. However, the function of hs-bindgen is limited. Moreover, hs-bindgen seems still a working experiment. I raised 2 issues about it which did not receive much attention from the author.
yvan-sraka/hs-bindgen-attribute#2, https://github.com/yvan-sraka/reflexive/issues/1

As for the second way, using the existing C bindings can simplify the process of creating Haskell bindings. However, there are also some potential downsides.

• Some C APIs are difficult to interact with haskell.
  For example, haskell can't deal with returned struct directly. struct opendal_result_read opendal_operator_blocking_read(struct opendal_operator_ptr ptr, const char *path); will be hard to use in haskell. We need to write some C codes to make it become void opendal_operator_blocking_read(struct opendal_operator_ptr ptr, const char *path, struct opendal_result_read* result);
• Less flexible
  If we want some features about haskell binding, we must wait it in C bindings.

Because of these reasons, I choose to create a C-compatible interface from the Rust library.

Also ask advices from maintainers.

Example

Exported API will like this.

import OpenDAL
import qualified Data.HashMap.Strict as HashMap

main :: IO ()
main = do
  op <- createOp "memory" HashMap.empty
    case op of
      Left e -> putStrLn $ "Failed to create operator, " ++ e
      Right op' -> do
        _ <- writeOp op' "key1" "value1"
        _ <- writeOp op' "key2" "value2"
        value1 <- readOp op' "key1"
        value2 <- readOp op' "key2"
        print value1
        print value2

Tests

I write a simple test testReadAndWriteToMemory, which can pass now.

[Xuanwo]

Mostly LGTM! This PR is good start! Are you willing to create a tracking issue of ongoing tasks? The first thing we should address is building the develop loop:

• Add contributing docs on how to setup the develop environment
• Add CI jobs on building and testing hs binding

And than we will need to implement more features, finally, we will handle the release process.

[silver-ymz]

issue #2465 is opened for tracking.