handle Length in non-selector position (Fixes #382)

New JsCore ADT representing JavaScript expressions.

New $SimpleMap op which applies a JsCore to each value.

Simple cases work, but many queries run into bugs in construction of groups
(see #403, #407).
delete commented line
Merge remote-tracking branch 'upstream/master' into ready/382
Merge pull request #427 from mossprescott/ready/382

handle Length in non-selector position (Fixes #382)