Allow variable refs as object keys {$key:value}

Users are often surprised by the requirement to parenthesize any
non-trivial object key expressions in object constructors.  E.g.,
{"a"+"b":1}.  This commit adds one more kind of key expression besides
literals and idents: variable references.

A common use case for this is jq programs as JSON templates to fill in
with variables computed from inputs or passed in on the command-line.
E.g., {some_key:$value}.  Now users can also use, e.g., {$key:$value}.

This and the restrictions on key and value expressions in object
constructors are now clarified a bit in the documentation.

docs/content/3.manual/manual.yml

@@ -549,13 +549,16 @@ sections:
           dictionaries or hashes), as in: `{"a": 42, "b": 17}`.
 
           If the keys are "identifier-like", then the quotes can be left
-          off, as in `{a:42, b:17}`.  Keys generated by expressions need
-          to be parenthesized, e.g., `{("a"+"b"):59}`.
+          off, as in `{a:42, b:17}`.  Variable references as key
+          expressions use the value of the variable as the key.  Key
+          expressions other than constant literals, identifiers, or
+          variable references, need to be parenthesized, e.g.,
+          `{("a"+"b"):59}`.
 
-          The value can be any expression (although you may need to
-          wrap it in parentheses if it's a complicated one), which gets
-          applied to the {} expression's input (remember, all filters
-          have an input and an output).
+          The value can be any expression (although you may need to wrap
+          it in parentheses if, for example, it contains colons), which
+          gets applied to the {} expression's input (remember, all
+          filters have an input and an output).
 
               {foo: .bar}
 
@@ -593,6 +596,16 @@ sections:
 
               {"stedolan": ["JQ Primer", "More JQ"]}
 
+          Variable references as keys use the value of the variable as
+          the key.  Without a value then the variable's name becomes the
+          key and its value becomes the value,
+
+              "f o o" as $foo | "b a r" as $bar | {$foo, $bar:$foo}
+
+          produces
+
+              {"f o o":"f o o","b a r":"f o o"}
+
         examples:
           - program: '{user, title: .titles[]}'
             input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}'