use Pratt's parser for expressions in the {log} block

This PR contains 2 commits: 
* 5c10af961f986a16b2a578e51fa084d6efae1cfc - support for multi-statement expressions
* 8fe2be85649ff1f4dd4f59706be8da49f6612835 - the actual switch of expression parsing for the {log} block

Please note that the second commit is actually removing more code than it is adding :-)