adds crystals parser code

lib/parser/expression.rb

@@ -0,0 +1,18 @@
+module Parser
+  module Expression
+    include Parslet
+    
+    rule(:value_expression) { call_site | basic_type }
+
+    rule(:expression) { (simple_return | while_do | conditional | operator_expression | call_site ) >> newline }
+
+    def delimited_expressions( delimit )
+      ( (delimit.absent? >> expression).repeat(1)).as(:expressions) >> delimit
+    end
+
+    rule(:expressions_do)     { delimited_expressions(keyword_do) }
+    rule(:expressions_else)   { delimited_expressions(keyword_else) }
+    rule(:expressions_end)    { delimited_expressions(keyword_end) }
+
+  end
+end