typing in RQL

It's quite big so we've decided to during a next CW sprint review it with @nsukami and several other people.

Merge requests that were accepted

Merge request that are waiting to be reviewed:

Notice: We'll also need to address this issue. RQL#5

Notice2: The following is a list (WIP) of rql's classes imported by Cubicweb. It was generated using this snippet:

{   'rql': {   'BadRQLQuery',
               'CoercionError',
               'RQLHelper',
               'RQLSyntaxError',
               'TypeResolverException',
               'nodes',
               'parse',
               'stmts',
               'utils'},
    'rql.analyze': {'ETypeResolver'},
    'rql.nodes': {   'ColumnAlias',
                     'Comparison',
                     'Constant',
                     'ETYPE_PYOBJ_MAP',
                     'Exists',
                     'Function',
                     'Not',
                     'Or',
                     'Relation',
                     'SubQuery',
                     'Variable',
                     'VariableRef',
                     'etype_from_pyobj'},
    'rql.stmts': {'Union', 'Select'},
    'rql.utils': {   'FunctionDescr',
                     'common_parent',
                     'decompose_b26',
                     'register_function',
                     'rqlvar_maker'}}

Name	Typed
BadRQLQuery	No need
CoercionError	No need
RQLHelper	Done
RQLSyntaxError	No need
TypeResolverException	No need
ETypeResolver	Not yet
nodes	Done
parse	Done
stmts	In progress
utils	Done