Fix various issues with subjects

All tests in crystal spec pass!

src/spectator/dsl/groups.cr

@@ -1,6 +1,7 @@
 require "../source"
 require "./builder"
 require "./tags"
+require "./values"
 
 module Spectator::DSL
   # DSL methods and macros for creating example groups.
@@ -89,13 +90,13 @@ module Spectator::DSL
           {{described_type}}
         end
 
-        private def _spectator_implicit_subject
         {% if described_type < Reference || described_type < Value %}
-            described_class.new
+          subject { described_class.new }
         {% else %}
+          private def subject
             described_class
-          {% end %}
           end
+        {% end %}
       {% else %}
         private def _spectator_implicit_subject
           {{what}}

src/spectator/dsl/values.cr

@@ -53,11 +53,15 @@ module Spectator::DSL
     # The block is evaluated only the first time the subject is referenced
     # and the return value is saved for subsequent calls.
     macro subject(name, &block)
-      subject {{block}}
+      {% raise "Block required for 'subject'" unless block %}
+      {% raise "Cannot use 'subject' inside of a test block" if @def %}
+      {% raise "Block argument count for 'subject' must be 0..1" if block.args.size > 1 %}
+
+      let({{name.id}}) {{block}}
 
       {% if name.id != :subject.id %}
-        def {{name.id}}
+        def subject
-          subject
+          {{name.id}}
         end
       {% end %}
     end
@@ -80,11 +84,15 @@ module Spectator::DSL
     # The block is evaluated once before the example runs
     # and the return value is saved for subsequent calls.
     macro subject!(name, &block)
-      subject! {{block}}
+      {% raise "Block required for 'subject!'" unless block %}
+      {% raise "Cannot use 'subject!' inside of a test block" if @def %}
+      {% raise "Block argument count for 'subject!' must be 0..1" if block.args.size > 1 %}
+
+      let!({{name.id}}) {{block}}
 
       {% if name.id != :subject.id %}
-        def {{name.id}}
+        def subject
-          subject
+          {{name.id}}
         end
       {% end %}
     end