Merge #12

12: doc(examples) Add the `greet` example. r=Hywan a=Hywan

```sh
$ ruby examples/greet.rb
$ Hello, Wasmer 💎!
```

😄 

Co-authored-by: Ivan Enderlin <[email protected]>

examples/greet.rb

@@ -0,0 +1,54 @@
+# coding: utf-8
+$LOAD_PATH.unshift File.expand_path("../../lib", __FILE__)
+
+require "wasmer"
+
+# Instantiates the module.
+file = File.expand_path "greet.wasm", File.dirname(__FILE__)
+bytes = IO.read file, mode: "rb"
+instance = Wasmer::Instance.new bytes
+
+# Set the subject to greet.
+subject = "Wasmer 💎".bytes
+length_of_subject = subject.length
+
+# Allocate memory for the subject, and get a pointer to it.
+input_pointer = instance.exports.allocate length_of_subject
+
+# Write the subject into the memory.
+memory = instance.memory.uint8_view input_pointer
+
+for nth in 0..length_of_subject - 1
+  memory[nth] = subject[nth]
+end
+
+# C-string terminates by NULL.
+memory[length_of_subject] = 0
+
+# Run the `greet` function. Give the pointer to the subject.
+output_pointer = instance.exports.greet input_pointer
+
+# Read the result of the `greet` function.
+memory = instance.memory.uint8_view output_pointer
+
+output = ""
+nth = 0
+
+while true
+  char = memory[nth]
+
+  if 0 == char
+    break
+  end
+
+  output += char.chr
+  nth += 1
+end
+
+length_of_output = nth
+
+puts output
+
+# Deallocate the subject, and the output.
+instance.exports.deallocate(input_pointer, length_of_subject)
+instance.exports.deallocate(output_pointer, length_of_output)

examples/greet.rs

@@ -0,0 +1,29 @@
+use std::ffi::{CStr, CString};
+use std::mem;
+use std::os::raw::{c_char, c_void};
+
+#[no_mangle]
+pub extern fn allocate(size: usize) -> *mut c_void {
+    let mut buffer = Vec::with_capacity(size);
+    let pointer = buffer.as_mut_ptr();
+    mem::forget(buffer);
+
+    pointer as *mut c_void
+}
+
+#[no_mangle]
+pub extern fn deallocate(pointer: *mut c_void, capacity: usize) {
+    unsafe {
+        let _ = Vec::from_raw_parts(pointer, 0, capacity);
+    }
+}
+
+#[no_mangle]
+pub extern fn greet(subject: *mut c_char) -> *mut c_char {
+    let subject = unsafe { CStr::from_ptr(subject).to_bytes().to_vec() };
+    let mut output = b"Hello, ".to_vec();
+    output.extend(&subject);
+    output.extend(&[b'!']);
+
+    unsafe { CString::from_vec_unchecked(output) }.into_raw()
+}