[monodoc] Changes to support new-style iOS documentation:

- Fixed bug in creating CREF strings.
- Refactored XML file processing for customization during new style iOS assembly. This was done through the new file source interface, which lets you use dependency injection to customize the process.
- Added the ability to rewrite CREF references when assembling docs that use native types. Also, the ability to handle dropped namespaces.
[mdoc] Changes to support new-style iOS API:

- the dropns flag now lets you define a namespace that is dropped in a new version of the assembly during update.
- the ntypes flag lets you tell the update command that certain methods are now using native types, and as such will attempt to match with old types in order to preserve documentation.
- the preserve flag will avoid removing members. This is to avoid dropping documentation for members that changed from one version of the framework to another new-style version (ie. iOS).
[mdoc] Added preserver.cs to build script so make compiles
[mdoc] Fixed an issue with type-file path resolution.
In certain cases, such as stubbing out namespace-less classes it was failing to produce the right paths.
[mdoc] Fixed issue with assembly versions being added incorrectly
[mdoc] Moved enumeration back into deferred mode.
This resolved an issue with the ecmadoc importer that assumed execution would be deffered in order to keep reading from the XmlReader
[mdoc] Style updates per pull request feedback
[mdoc] Changed 'old' and 'new' to 'classic' and 'unified' per feedback.
Test XmlSerialization of class with XmlSchemaProvider with IsAny.
Covers #11916.
Fixed XmlSerializer to handle attribute XmlSchemePrivider.IsAny.
XmlSerializer no longer outputs a root element with class name when the class has the attribute XmlSchemeProvider and IsAny is true.
Fixes #11916
Merge pull request #1212 from esdrubal/isany

Fixed XmlSerializer to handle attribute XmlSchemePrivider.IsAny.
Added test BaseClassWithoutParameterlessConstructor.
Covers #6913
[utils] Improve error message in case of thread suspend fails.
Xml Serialization of Base class w/o a parameterless constructor.
Removed validation code that did not allowed serialization of base classes without a parameterless constructor.
Fixes #6913.
Merge pull request #1214 from esdrubal/xmlbase

Xml Serialization of Base class w/o a parameterless constructor.
Fix a warning.
[bcl] Add a missing check to TypeBuilder.CreateType (). Fixes #22059.
Removing excess line with debug Console.WriteLine
Merge pull request #1216 from AndreyAkinshin/master

Removing excess line with debug Console.WriteLine
[mcs] More warnings for lifted nullable operators working on value types