Some refactors in specs for not requiring prelude, and some typos fixes

spec/compiler/codegen/def_spec.cr

@@ -19,15 +19,28 @@ describe "Code gen: def" do
   end
 
   it "call external function 'putchar'" do
-    run("require \"c\"; require \"io\"; C.putchar '\\0'").to_i.should eq(0)
+    run("
+      lib C
+        fun putchar(c : Char) : Char
+      end
+      C.putchar '\\0'
+      ").to_i.should eq(0)
   end
 
   it "uses self" do
     run("struct Int; def foo; self + 1; end; end; 3.foo").to_i.should eq(4)
   end
 
   it "uses var after external" do
-    run("require \"c\"; require \"io\"; a = 1; C.putchar '\\0'; a").to_i.should eq(1)
+    run("
+      lib C
+        fun putchar(c : Char) : Char
+      end
+
+      a = 1
+      C.putchar '\\0'
+      a
+      ").to_i.should eq(1)
   end
 
   it "allows to change argument values" do

spec/compiler/codegen/responds_to_spec.cr

@@ -18,15 +18,15 @@ describe "Codegen: responds_to?" do
     run("(1 == 1 ? 1 : 'a').responds_to?(:\"foo\")").to_b.should be_false
   end
 
-  it "codegens is_a? with nilable gives true" do
+  it "codegens responds_to? with nilable gives true" do
     run("struct Nil; def foo; end; end; (1 == 1 ? nil : Reference.new).responds_to?(:foo)").to_b.should be_true
   end
 
-  it "codegens is_a? with nilable gives false becuase other type 1" do
+  it "codegens responds_to? with nilable gives false becuase other type 1" do
     run("(1 == 1 ? nil : Reference.new).responds_to?(:foo)").to_b.should be_false
   end
 
-  it "codegens is_a? with nilable gives false becuase other type 2" do
+  it "codegens responds_to? with nilable gives false becuase other type 2" do
     run("class Reference; def foo; end; end; (1 == 2 ? nil : Reference.new).responds_to?(:foo)").to_b.should be_true
   end
 end