Properly call host machine functions defined via imports

[alexdovzhanyn]

This PR adds functionality to properly run Wasm modules that call functions that are imported from the host machine (i.e. calling IO.puts from within a wasm function). If a module relies on imports, those imports must be passed in along with the module in order for it to correctly run, otherwise the VM will crash. Validation of imports being passed in will be added in #7

Using imports:

Given a WebAssembly file which calls imported functions:

(module
  (import "math" "add100" (func $add100 (param f32) (result f32)))
  (import "env" "consoleLog" (func $consoleLog (param f32)))
  (export "getSqrt" (func $getSqrt))
  (func $getSqrt (param f32) (result f32)
    get_local 0
    call $consoleLog

    get_local 0
    call $add100
    tee_local 0
    call $consoleLog

    get_local 0
    f32.sqrt
  )
)

An imports map must be passed in to the load_file or load function. This map will have 2 layers; the first layer is the "module name", as defined in the Wasm file, and the second layer is the function name. The keys of the map must be strings.

{:ok, pid} = WaspVM.start()

imports = %{
  "env" => %{
    "consoleLog" => fn x -> IO.puts x end
  },
  "math" => %{
    "add100" => fn x -> x + 100 end
  }
}

WaspVM.load_file(pid, "test/fixtures/wasm/log.wasm", imports)
WaspVM.execute(pid, "getSqrt", [25])