Replace .ToLambda() conversions with static typing

[stakx]

This is the first out of several refactorings that attempt to get rid (as much as possible) of chained expression transforms such as .PartialMatcherAwareEval().ToLambda() which are found all over the codebase. They are problematic because it's not immediately clear when these transforms are needed, and which of them are needed for what reasons.

.ToLambda() is a mix-up of two separate concerns:

1. Perform a downcast from Expression to LambdaExpression
2. Strip a type-cast (if present) from the lambda's body

As it turns out, (1) becomes superfluous if we simply re-declare certain members as LambdaExpression instead of Expression. Incidentally, this makes visible the heretofore unstated expectation that MethodCall.SetupExpression be a lambda expression.

ToLambda is then renamed to StripConversion to better reflect its remaining function (2).