Implemented overloaded versions of Parse and TryParse functions for BigInteger.

Implementation is heavily inspired by Int32.
Added unit tests for the new functions.
Merge branch 'BigIntegerParse'
Merge remote-tracking branch 'upstream/master'
Cleanup and better comment.
Implemented DaysInYearBeforeMinSupportedYear for all Calendars that should have it in .Net 4.5
[corlib] Fix implementation of Tuple hash generator

The Tuples.cs code generator dies with:

Unhandled Exception:
System.NotImplementedException: The requested feature is not implemented.
  at TupleGen.computeHash (Int32 destVar, Int32 start, Int32 count) [0x00000] in <filename unknown>:0
  at TupleGen.WriteHash (Int32 destVar, Int32 start, Int32 count) [0x00000] in <filename unknown>:0
  at TupleGen.Main () [0x00000] in <filename unknown>:0

This is caused by the incorrect definition of WriteHash(), which
instead of recursively calling itself is calling computeHash().

Thanks to Marek Safar for reporting the issue in
https://github.com/fsharp/fsharp/issues/188#issuecomment-34351850
Merge pull request #895 from ranma42/fix-tuple-hash-codegen

[corlib] Fix implementation of Tuple hash generator
Merge pull request #892 from initram/master

Implemented Parse() and TryParse() for System.Numerics.BigInteger and DaysInYearBeforeMinSupportedYear for Calendars
Revert "Merge pull request #829 from symform/httpwebreq-async-ssl"

This reverts commit 35899c07fff7f7e4c6533364091bbac6ce86fdfd, reversing
changes made to 80166fd02fadc470e40b59df0c8c65c19945ed21.
[corlib] Mark awaited task exception observed when GetResult does the wait
[sgen] Fix overflow occurring with large object allocations.  Fixes #17590.
[sgen] Hint OOM exception branch as not taken.
[sgen] new T[-1] must throw OverflowException, not OOM.

Fixes failure in mini/arrays.cs.
[runtime] Remove overly pessimistic test.

The test asserts that we can't allocate int arrays with 2G elements.
With SGen on 64 bits, however, we can, which is a feature, not
a bug.
[system.xml] Support more primitive types in XmlReader::ReadContentAs
[arm] Use d0/d1 as fp scratch regs on non-hard float platforms. Hopefully fixes #17597.
[system.xml] Add more invalid argument checks
[system] Updates to WebUtility decoders and encoders
[NTLM]: Add new 'NtmlSettings' class to hold the 'DefaultAuthLevel' property.

When the NTLM sources are compiled into System.dll, then we need to store the
configuration variables there and peek/poke it in Mono.Security.
[mcs] Don't report any not applicable operator errors during constant folding.