Generic query expression parsing and AST manipulation.

QUERY EXPRESSIONS

Query expressions are a variation on Datomic Pull Syntax
http://docs.datomic.com/pull.html more suitable for generic client/server
state transfer. It's important to note the query expression syntax is
*not* a strict superset of Datomic Pull.

A query expression is composed of EDN values. The grammar for query
expressions follows:

QueryRoot      := EdnVector(QueryExpr*)
PlainQueryExpr := (EdnKeyword | IdentExpr | JoinExpr)
QueryExpr      := (PlainQueryExpr | ParamExpr)
IdentExpr      := EdnVector2(Keyword, EdnValue)
ParamExpr      := EdnList2(PlainQueryExpr | EdnSymbol, ParamMapExpr)
ParamMapExpr   := EdnMap(Keyword, EdnValue)
JoinExpr       := EdnMap((Keyword | IdentExpr), (QueryRoot | UnionExpr | RecurExpr))
UnionExpr      := EdnMap(Keyword, QueryRoot)
RecurExpr      := ('... | Integer)

Note most of the api expects a QueryRoot not a QueryExpr.

QUERY EXPRESSION AST FORMAT

Given a QueryExpr you can get the AST via om.next.impl.parser/expr->ast.
The following keys can appear in the AST representation:

{:type         (:prop | :join | :call | :root | :union | :union-entry)
 :key          (EdnKeyword | EdnSymbol | IdentExpr)
 :dispatch-key (EdnKeyword | EdnSymbol)
 :union-key    EdnKeyword
 :query        (QueryRoot | RecurExpr)
 :params       ParamMapExpr
 :children     EdnVector(AST)
 :component    Object
 :target       EdnKeyword}

:query and :params may or may not appear. :type :call is only for
mutations.