Add support for String.Length property in expressions.

[sflanker]

See test cases:

Invio.QueryProvider.MySql/test/Invio.QueryProvider.MySql.Test/MySqlQueryableTest.cs

Lines 136 to 138 in 03146ab

	[Fact(Skip = "TODO: Not Supported")]
	public override void Select_StringLength() {
	}

Invio.QueryProvider.MySql/test/Invio.QueryProvider.MySql.Test/MySqlQueryableTest.cs

Lines 148 to 150 in 03146ab

	[Fact(Skip = "TODO: Not Supported")]
	public override void Where_String_Length() {
	}

This expression should work in all contexts: Select/Where/OrderBy