WASI fd_write to stdout is buffered and should be unbuffered

[kanaka]

Describe the bug

The current implementation of fd_write in wasmer 0.3.0 is buffered when writing to stdout (fd 1). The fd_write WASI API is designed to be similar to the POSIX write/writev calls and those write unbuffered output to stdout. In wasmtime fd_write is unbuffered. In my specific case, this prevents fd_write from being used to print a prompt that doesn't have a newline.

Steps to reproduce

The following wat code:

(module
  (import "wasi_unstable" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
  (import "wasi_unstable" "fd_read" (func $fd_read (param i32 i32 i32 i32) (result i32)))

  (memory 10)
  (export "memory" (memory 0))

  ;; prompt __wasi_ciovec_t struct
  (data (i32.const 0) "\08\00\00\00") ;; buf: pointer to prompt string
  (data (i32.const 4) "\02\00\00\00") ;; buf_len: 2 characters
  ;; string
  (data (i32.const 8) "> ")

  ;; read buf __wasi_ciovec_t struct
  (data (i32.const 16) "\18\00\00\00") ;; buf: pointer to hello string
  (data (i32.const 20) "\64\00\00\00") ;; buf_len: 100 characters max
  ;; string
  (data (i32.const 24) "\00") ;; buf (of 100 characters) to hold read in string

  (func $main (local i32)
    (drop (call $fd_write (i32.const 1) (i32.const 0) (i32.const 1) (i32.const 256)))
    (drop (call $fd_read (i32.const 0) (i32.const 16) (i32.const 1) (i32.const 256)))
    (drop (call $fd_write (i32.const 0) (i32.const 16) (i32.const 1) (i32.const 256)))
  )

  ;;(start $main)
  (export "_start" $main)
)

results in the following behavior:

$ ~/.wasmer/bin/wasmer run wasitests/echo.wasm 
user input
> 

Expected behavior

The prompt should be printed before the user input. Here is the correct behavior from wasmtime:

$ wasmtime wasitests/echo.wasm
> user input
user input