Remove or cache some unnecessary allocations

[stephentoub]

I noticed a few places where allocations were occurring unnecessary:

• Across several of the immutable and XML collection types, the ICollection.CopyTo implementations were calling Array.SetValue in a loop; the second parameter to SetValue is a params array, so each iteration of the loop was resulting in allocating a new array... I've lifted that implicit allocation out to be an explicit one before the loop.
• In a couple of places in the XML library and the metadata reader, string.Trim* was being used, either with an array of the same characters unnecessarily being allocated each time, or an implicitly allocated array of constant chars to fill a params array parameter. I've replaced those with statically cached arrays.
• In a couple of places in the XML library, a string was being constructed around a single character via creating a new char array; I've replaced that with usage of string's ctor that takes a character and a count, avoiding the unnecessary char[] allocation.

[stephentoub]

ps In the case of the loop around SetValue, I contemplated adding if checks to avoid the allocation in the case where the collection being copied from is empty. I'd initially opted not to, under the assumption that copying from an empty collection using ICollection.CopyTo is uncommon. But looking at it again I think I will. Closing the pull request; I'll reissue it.

[karelz]

@Anipik one more ... just FYI