For loop/environment fixes

There was a problem where the body of a for loop was treated like a
function and got an entirely new environment. This meant that you could
not, for instance, modify a variable in a loop and keep the changed
value after the loop finished executing.

Also, for consistency, switch to storing only fully evaluated values in
the environment.

lib/ahem/ast/assignment.rb

 
   def execute(env)
     if env.get(@name.name)
-      env.set(@name.name, @value)
+      env.set(@name.name, @value.execute(env))
     else
       raise "Invalid assignment to undeclared variable #{@name.name}"
     end

lib/ahem/ast/for_loop.rb

 
   def execute(env)
     @iterable.execute(env).each do |i|
-      _env = Environment.new
-      _env.set(@iterator.name, i)
-      @block.statements.each { |statement| statement.execute(_env) }
+      env.set(@iterator.name, i)
+      @block.statements.each { |statement| statement.execute(env) }
     end
   end
 end

lib/ahem/ast/variable_declaration.rb

 
   def execute(env)
     if env.data[@name.name].nil?
-      env.set(@name.name, @value)
+      env.set(@name.name, @value.execute(env))
     else
       raise "Invalid declaration of previously declared variable #{@name.name}"
     end

spec/ast/assignment_spec.rb

     AST::Assignment.new(AST::Identifier.new('x'), AST::Number.new(6.0)).execute(
       env
     )
-    expect(env.get('x').execute(env)).to eq(6.0)
+    expect(env.get('x')).to eq(6.0)
   end
 
   it 'raises for an undeclared variable' do

spec/ast/for_loop_spec.rb

         .execute(Environment.new)
     }.to output("1.0\n2.0\n3.0\n").to_stdout
   end
+
+  it 'does not create a new scope inside the loop' do
+    env = Environment.new
+    AST::VariableDeclaration.new(AST::Identifier.new('x'), AST::Number.new(0.0))
+      .execute(env)
+    result =
+      AST::ForLoop.new(
+        AST::Identifier.new('y'),
+        AST::Array.new(
+          [AST::Number.new(1.0), AST::Number.new(2.0), AST::Number.new(3.0)]
+        ),
+        AST::Block.new(
+          [
+            AST::Assignment.new(
+              AST::Identifier.new('x'),
+              AST::Binary.new(
+                AST::Operators::ADD,
+                AST::Identifier.new('x'),
+                AST::Identifier.new('y')
+              )
+            )
+          ]
+        )
+      )
+        .execute(env)
+    expect(env.get('x')).to eq(6.0)
+  end
 end

spec/ast/variable_declaration_spec.rb

     env = Environment.new
     AST::VariableDeclaration.new(AST::Identifier.new('x'), AST::Number.new(5.0))
       .execute(env)
-    expect(env.get('x').execute(env)).to eq(5.0)
+    expect(env.get('x')).to eq(5.0)
   end
 
   it 'raises if a variable is already defined' do