Speedup Parsers.validID

It turned up in profiling sessions. Previously, it used parser combinators
which are somewhat slow especially when the JVM is still cold.

The grammar for ID is simple enough to afford this handwritten parser.

internal/util-complete/src/main/scala/sbt/internal/util/complete/Parsers.scala

@@ -12,15 +12,17 @@ import Parser._
 import java.io.File
 import java.net.URI
 import java.lang.Character.{
-  getType,
-  MATH_SYMBOL,
-  OTHER_SYMBOL,
+  CURRENCY_SYMBOL,
   DASH_PUNCTUATION,
-  OTHER_PUNCTUATION,
+  MATH_SYMBOL,
   MODIFIER_SYMBOL,
-  CURRENCY_SYMBOL
+  OTHER_PUNCTUATION,
+  OTHER_SYMBOL,
+  getType
 }
 
+import scala.annotation.tailrec
+
 /** Provides standard implementations of commonly useful [[Parser]]s. */
 trait Parsers {
 
@@ -313,6 +315,16 @@ object DefaultParsers extends Parsers with ParserMain {
     apply(p)(s).resultEmpty.isValid
 
   /** Returns `true` if `s` parses successfully according to [[ID]].*/
-  def validID(s: String): Boolean = matches(ID, s)
+  def validID(s: String): Boolean = {
+    // Handwritten version of `matches(ID, s)` because validID turned up in profiling.
+    def isIdChar(c: Char): Boolean = Character.isLetterOrDigit(c) || (c == '_')
+    @tailrec def isRestIdChar(cur: Int, s: String, length: Int): Boolean =
+      if (cur < length)
+        isIdChar(s.charAt(cur)) && isRestIdChar(cur + 1, s, length)
+      else
+        true
+
+    !s.isEmpty && Character.isLetter(s.charAt(0)) && isRestIdChar(1, s, s.length)
+  }
 
 }