Add extensibility point for LINQ expression tree compilation

[stakx]

As contemplated in #504 (comment).

This would enable users to configure Moq to use their own LINQ expression tree compilation strategies, such as:

• add a caching layer, as originally proposed in Moq 64bit bad performance #188 (comment);
• use a different (possibly faster) library for compilation, as suggested in Find out why Moq's performance has become worse since version approx. 4.5 #504 (comment).

It would seem like a nice feature to merge, since it enables some new options and is trivial to implement.