Reserve A Bit of Scratch Space In The RecordBuffer

This improves the performance of appending strings to the bitstream writer somewhat by giving us some room to work with.

Author: CodaFi

Sources/TSCUtility/BitstreamReader.swift

@@ -148,6 +148,10 @@ private struct BitstreamReader {
     }
   }
 
+  /// Computes a non-owning view of a `BitcodeElement.Record` that is valid for
+  /// the lifetime of the call to `body`.
+  ///
+  /// - Warning: If this function throws, the `body` block will not be called.
   mutating func withAbbreviatedRecord(
     _ abbrev: Bitstream.Abbreviation,
     body: (BitcodeElement.Record) throws -> Void

Sources/TSCUtility/BitstreamWriter.swift

@@ -338,6 +338,7 @@ extension BitstreamWriter {
 
         fileprivate init() {
             self.values = []
+            self.values.reserveCapacity(8)
         }
 
         fileprivate init<CodeType>(recordID: CodeType)
@@ -367,6 +368,7 @@ extension BitstreamWriter {
         }
 
         public mutating func append(_ string: String) {
+            self.values.reserveCapacity(self.values.capacity + string.utf8.count)
             for byte in string.utf8 {
                 values.append(UInt32(byte))
             }