refactor: remove expressions parsing from peg grammar

[marclaval]

:+1:

[PK1A]

I was kind of expecting that we can remove more (and probably can) but further cleanup would require:

• understanding why do we need the JSObjectRef rule
• deciding if we want to keep reserved words checking on the peg level

Anyway, even with those first changes we are dropping size of the generated peg from 16K -> 9K

[coveralls]

Coverage remained the same when pulling f1d7fbf5feadc99ee71f5b28a79aeec64323ccd1 on PK1A:peg_cleanup into aa6a7b238182428de4c1e91f57d4c914f0511ed0 on ariatemplates:master.