Add Array.join for joining Array values

The method Array.join can be used to join values in an Array together
using a separator. This method is only available if the values in an
Array implement the ToString trait.

runtime/src/std/array.inko

@@ -13,7 +13,6 @@
 # indexes, in which case the value will be accessed starting at the back of the
 # Array. This means that -1 will access the last value, -2 the value before
 # that, etc.
-
 import std::conversion::ToArray
 import std::index::(Index, SetIndex)
 import std::length::Length

runtime/src/std/array/extensions.inko

@@ -4,8 +4,10 @@
 # These types and methods are defined separately as otherwise `std::iterator`
 # would depend on `std::array` while `std::array` would depend on
 # `std::iterator`.
+import std::conversion::ToString
 import std::format::(self, Formatter, Inspect)
 import std::iterator::(self, Iterator)
+import std::string_buffer::StringBuffer
 
 impl Array!(T) {
   # Returns an `Iterator` that iterates over all values in `self`.
@@ -26,6 +28,31 @@ impl Array!(T) {
       self[index]
     }
   }
+
+  # Joins the values of `self` together using the separator.
+  #
+  # # Examples
+  #
+  # Joining an empty `Array`:
+  #
+  #     Array.new.join(',') # => ''
+  #
+  # Joining an `Array` with multiple values:
+  #
+  #     Array.new(10, 20, 30).join(',') # => '10,20,30'
+  def join(separator: String) -> String where T: ToString {
+    let buffer = StringBuffer.new
+
+    each_with_index do (value, index) {
+      index.positive?.if_true {
+        buffer.push(separator)
+      }
+
+      buffer.push(value.to_string)
+    }
+
+    buffer.to_string
+  }
 }
 
 impl Inspect for Array!(T) {

runtime/tests/test/std/test_array.inko

@@ -228,3 +228,25 @@ test.group('std::array::Array.inspect') do (g) {
     assert.equal(Array.new(10, 20).inspect, 'Array { 10, 20 }')
   }
 }
+
+test.group('std::array::Array.join') do (g) {
+  g.test('Joining values in an empty Array') {
+    assert.equal(Array.new.join(','), '')
+  }
+
+  g.test('Joining a single value') {
+    assert.equal(Array.new(10).join(','), '10')
+  }
+
+  g.test('Joining multiple values') {
+    assert.equal(Array.new(10, 20, 30).join(','), '10,20,30')
+  }
+
+  g.test('Joining multiple values using multiple characters as the separator') {
+    assert.equal(Array.new(10, 20, 30).join('::'), '10::20::30')
+  }
+
+  g.test('Joining multiple values using an empty separator') {
+    assert.equal(Array.new(10, 20, 30).join(''), '102030')
+  }
+}