Add Type annotations and remove Symbol usage

Remove Ruby's *symbols everywhere* approach to define getter/setters
or properties. Crystal's parser and macros do not require symbols
for these options.

Also add type annotations to some elements aiming to improve quality
of the documentation generated (more accurate expected types).

src/radix/node.cr

@@ -27,10 +27,10 @@ module Radix
   # # => [3, 2, 1]
   # ```
   class Node
-    getter :key
-    getter? :placeholder
-    property! :payload
-    property :children
+    getter key : String
+    getter? placeholder : Bool
+    property! payload
+    property children : Array(Node)
 
     # Returns the priority of the Node based on it's *key*
     #
@@ -54,7 +54,7 @@ module Radix
     # Node.new(":query").priority
     # # => 1
     # ```
-    getter :priority
+    getter priority : Int32
 
     # Instantiate a Node
     #

src/radix/result.cr

@@ -13,8 +13,8 @@ module Radix
   # A Result is also used recursively by `Tree#find` when collecting extra
   # information like *params*.
   class Result
-    getter :params
-    getter! :payload
+    getter params
+    getter! payload
 
     # :nodoc:
     def initialize
@@ -35,7 +35,7 @@ module Radix
     # result.found?
     # # => true
     # ```
-    def found?
+    def found? : Bool
       payload? ? true : false
     end
 
@@ -60,7 +60,7 @@ module Radix
     # result.key
     # # => ""
     # ```
-    def key
+    def key : String
       return @key if @key
 
       key = String.build { |io|

src/radix/tree.cr

@@ -30,7 +30,7 @@ module Radix
     # Returns the root `Node` element of the Tree.
     #
     # On a new tree instance, this will be a placeholder.
-    getter :root
+    getter root : Node
 
     def initialize
       @root = Node.new("", placeholder: true)
@@ -397,7 +397,7 @@ module Radix
     end
 
     # :nodoc:
-    private def deprecation(message)
+    private def deprecation(message : String)
       STDERR.puts message
       STDERR.flush
     end