Add Deprecated annotation to prelude and docs

src/annotations.cr

@@ -0,0 +1,11 @@
+# This annotations marks methods as deprecated.
+#
+# It receives an optional `StringLiteral` as single argument containing a deprecation notice.
+#
+# ```
+# @[Deprecated("Use #bar instead")]
+# def foo
+# end
+# ```
+annotation Deprecated
+end

src/docs_main.cr

@@ -2,6 +2,7 @@
 # It, for example, doesn't include API for the compiler, but does include
 # the fictitious API for the Crystal::Macros module.
 
+require "./annotations"
 require "./big"
 require "./compiler/crystal/macros"
 require "./crypto/**"

src/prelude.cr

@@ -28,6 +28,7 @@ require "indexable"
 require "string"
 
 # Alpha-sorted list
+require "annotations"
 require "array"
 require "atomic"
 require "bool"