stdlib: add a function to squeeze a number in a given range from a hash value

This function mixes the bits in the hash value, which improves Dictionary
performance for keys with bad hashes.

PrecommitBenchmark changes with greater than 7% difference:

``````````Dictionary2`,```1456.00`,```1508.00`,```1502.00`,````624.00`,````607.00`,````592.00`,`864.00`,``145.9%
``````````Dictionary3`,```1379.00`,```1439.00`,```1408.00`,````585.00`,````567.00`,````552.00`,`827.00`,``149.8%
````````````Histogram`,````850.00`,````849.00`,````851.00`,```1053.00`,```1049.00`,```1048.00`,`199.00`,``-19.0%
````````````````Prims`,```1999.00`,```2005.00`,```2018.00`,```1734.00`,```1689.00`,```1701.00`,`310.00`,```18.4%
``````````StrSplitter`,```2365.00`,```2334.00`,```2316.00`,```1979.00`,```1997.00`,```2000.00`,`337.00`,```17.0%
```````````````TwoSum`,```1551.00`,```1568.00`,```1556.00`,```1771.00`,```1741.00`,```1716.00`,`165.00`,```-9.6%

Regressions are in benchmarks that use `Int` as dictionary key: we are just
doing more work than previously (hashing an `Int` was an identity function).

rdar://17962402


Swift SVN r21142

stdlib/unittest/StdlibUnittest.swift.gyb

@@ -235,6 +235,7 @@ public func assertionFailure() -> AssertionResult {
 public func expectTrue(
     actual: ${BoolType},
     stackTrace: SourceLocStack? = nil,
+    _ collectMoreInfo: (()->String)? = nil,
     file: String = __FILE__, line: UWord = __LINE__
 ) {
   if !actual {
@@ -243,6 +244,7 @@ public func expectTrue(
     _printStackTrace(stackTrace)
     println("expected: true")
     println("actual: \(actual)")
+    if collectMoreInfo != nil { println(collectMoreInfo!()) }
     println()
   }
 }
@@ -250,6 +252,7 @@ public func expectTrue(
 public func expectFalse(
     actual: ${BoolType},
     stackTrace: SourceLocStack? = nil,
+    _ collectMoreInfo: (()->String)? = nil,
     file: String = __FILE__, line: UWord = __LINE__
 ) {
   if actual {
@@ -258,6 +261,7 @@ public func expectFalse(
     _printStackTrace(stackTrace)
     println("expected: false")
     println("actual: \(actual)")
+    if collectMoreInfo != nil { println(collectMoreInfo!()) }
     println()
   }
 }