Adding warning for running from bare cmd
Fix misspelled Utf8NullTerminatedStringStartsWithAsciiPrefix
Fix comment typos in MemoryBlock
Fix misspelled test Utf8NullTerminatedStringStartsWithAsciiPrefix
Fast implementation of Hamming weight

The target method: System.Reflection.Internal.BitArithmetic.CountBits
Old implementation uses 24 arithmetic operations
New fast implementation uses 12 arithmetic operations
System.Xml.XDocument Typos

System.Xml.XDocument Typos removed tabs in typo fixes
System.Xml.XmlDocument Typos

System.Xml.XmlDocument removed tabs from typo fixes
System.Xml.XPath Typos

System.Xml.XPath removed tabs in typo fixes

System.Xml.XPath undo wrong typo fix
System.Xml.XPath.XmlDocument Typos
System.Reflection.Metadata Typos
System.Numerics.Vectors Typos

System.Numerics.Vectors removed tabs in typo fixes
System.Collections.Immutable Typos

System.Collections.Immutable removed tabs from typo fixes
Remove unnecessary allocations from Keys, Values, and ContainsValue members
Merge pull request #155 from AndreyAkinshin/FastHammingWeight

Fast implementation of Hamming weight
Update links to developer guide in error messages to new location
on the dotnet/corefx wiki.
Add System.Linq.Parallel Product and Test Code
Merge branch 'master' of https://github.com/dotnet/corefx
Cause build.cmd to set errorlevel to 1 on failed prereqs
Cause appveyor to run CI build in Dev12 developer command prompt.
Update README.md to include PLinq description.

Reviewed by @joshfree and @krwq.