Proposal edits, append-in-place additions

Author: glessard

proposals/nnnn-outputspan.md

@@ -53,15 +53,15 @@ extension Array {
 }
 ```
 
-We will also extend `String` and `InlineArray` in a similar manner.
+We will also extend `String`, `UnicodeScalarView` and `InlineArray` with similar initializers, and add append-in-place operations where appropriate.
 
 #### `@lifetime` attribute
 
 Some of the API presented here must establish a lifetime relationship between a non-escapable returned value and a callee binding. This relationship will be illustrated using the `@lifetime` attribute recently [pitched][PR-LifetimeAnnotations] and [formalized][Forum-LifetimeAnnotations]. For the purposes of this proposal, the lifetime attribute ties the lifetime of a function's return value to one of its input parameters.
 
 Note: The eventual lifetime annotations proposal may adopt a syntax different than the syntax used here. We expect that the Standard Library will be modified to adopt an updated lifetime dependency syntax as soon as it is finalized.
 
-## Detailed Design
+## <a name="design"></a>Detailed Design
 
 #### OutputSpan
 
@@ -563,18 +563,45 @@ extension Array {
     capacity: Int,
     initializingWith initializer: (inout OutputSpan<Element>) throws(E) -> Void
   ) throws(E)
+  
+  public mutating func append<E: Error>(
+    addingCapacity: Int,
+    initializingWith initializer: (inout OutputSpan<Element>) throws(E) -> Void
+  ) throws(E)
 }
 
 extension String {
   public init<E: Error>(
-    capacity: Int,
+    utf8Capacity: Int,
     initializingUTF8With initializer: (inout OutputSpan<UTF8.CodeUnit>) throws(E) -> Void
   )
 
   public init<E: Error>?(
-    capacity: Int,
+    utf8Capacity: Int,
     initializingValidUTF8With initializer: (inout OutputSpan<UTF8.CodeUnit>) throws(E) -> Void
   ) throws(E)
+  
+  public mutating func append<E: Error>(
+    addingUTF8Capacity: Int,
+    initializingUTF8With initializer: (inout OUtputSpan<UTF8.CodeUnit>) throws(E) -> Void
+  ) throws(E)
+}
+
+extension UnicodeScalarView {
+  public init<E: Error>(
+    utf8Capacity: Int,
+    initializingUTF8With initializer: (inout OutputSpan<UTF8.CodeUnit>) throws(E) -> Void
+  )
+
+  public init<E: Error>?(
+    utf8Capacity: Int,
+    initializingValidUTF8With initializer: (inout OutputSpan<UTF8.CodeUnit>) throws(E) -> Void
+  ) throws(E)
+  
+  public mutating func append<E: Error>(
+    addingUTF8Capacity: Int,
+    initializingUTF8With initializer: (inout OUtputSpan<UTF8.CodeUnit>) throws(E) -> Void
+  ) throws(E)
 }
 
 extension InlineArray {