Remove System.Collections.Immutable.xml from under the nuget folder
The documentation in this file is written by technical writers and does not come from the xml comments in the library. The nuspec file uses the xml generated by the library and not this one. Removing the file.
Merge branch 'master' of https://github.com/dotnet/corefx
Significant enumeration perf improvements for immutable lists, dictionaries, and sets

These are the approximate improvements I see on my machine when enumerating various
immutable collection types of length 0, 1, and 1000.  The numbers are old
time divided by new time...

ImmutableList:
    Empty - 1.6x
    Single element - 2.2x
    1000 elements - 2.9x

ImmutableHashSet:
    Empty - 1.3x
    Single element - 1.7x
    1000 elements - 1.6x

ImmutableSortedSet:
    Empty - 1.9x
    Single element - 2.5x
    1000 elements - 2.7x

ImmutableDictionary:
    Empty - 1.2x
    Single element - 1.6x
    1000 elements - 1.4x

ImmutableSortedDictionary:
    Empty - 2x
    Single element - 2.5x
    1000 elements - 2.75x
Address CR feedback on previous commit
Merge pull request #135 from stephentoub/immutable_enum_perf

Perf improvements for enumerating immutable collections