Fix some clippy warnings

[jacwah]

In response to #458.

I did not fix two of clippy's warnings:

warning: returning the result of a let binding from a block. Consider returning the expression directly.
   --> src/renderer/html_handlebars/hbs_renderer.rs:137:9
    |
137 |         rendered
    |         ^^^^^^^^
    |
    = note: #[warn(let_and_return)] on by default
note: this expression can be directly returned
   --> src/renderer/html_handlebars/hbs_renderer.rs:135:24
    |
135 |         let rendered = add_playpen_pre(&rendered, playpen_config);
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.165/index.html#let_and_return

I think the code is more readable as it is currently than clippy's suggestion. Should I add
#[cfg_attr(feature = "cargo-clippy", allow(let_and_return))] or are warnings okay here?

warning: large size difference between variants
   --> src/lib.rs:104:5
    |
104 | /     error_chain!{
105 | |         foreign_links {
106 | |             Io(::std::io::Error);
107 | |             HandlebarsRender(::handlebars::RenderError);
...   |
117 | |         }
118 | |     }
    | |_____^
    |
    = note: #[warn(large_enum_variant)] on by default
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.165/index.html#large_enum_variant
help: consider boxing the large fields to reduce the total size of the enum
    |
113 | err : Box<$ foreign_link_error_path> ) {
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Not sure what to do about this one. Boxing the errors seems more invasive, so I'll leave that decision to someone else. I don't see why any of these variants should be large though.

[budziq]

lgtm. Thanks for the PR @jacwah !

[azerupi]

Thanks for the fixes! :)

[projektir]

Should I add
#[cfg_attr(feature = "cargo-clippy", allow(let_and_return))] or are warnings okay here?

I think I would. I have been in that part of the code and I agree it's more readable the way it is, but overall the clippy rule is good. We don't want "accepted" warnings cluttering up things, I don't think.

Not sure about the Box one, though.

Generally, having "accepted" warnings is bad in my opinion (then you have to mentally exclude them every time you run clippy again) so finding some way to make them go away is preferred.

[jacwah]

Regarding large_enum_variant: handlebars::TemplateError is the culprit here. Boxing it halves the size of mdbook::errors::Error from 256 to 128 bytes on my x86_64 machine.

Type	mem::size_of
io::Error	16
RenderError	96
TemplateError	224
FromUtf8Error	40
mdbook::errors::Error	256

After boxing TemplateError:

Type	mem::size_of
mdbook::errors::Error	128

Boxing RenderError too makes only a marginal difference:

Type	mem::size_of
mdbook::errors::Error	112

Boxing can be done transparently like this:

--- a/src/lib.rs
+++ b/src/lib.rs
@@ -104,8 +104,8 @@ pub mod errors {
     error_chain!{
         foreign_links {
             Io(::std::io::Error);
             HandlebarsRender(::handlebars::RenderError);
-            HandlebarsTemplate(::handlebars::TemplateError);
+            HandlebarsTemplate(Box<::handlebars::TemplateError>);
             Utf8(::std::string::FromUtf8Error);
         }
 
@@ -116,4 +116,10 @@ pub mod errors {
             }
         }
     }
+
+    impl From<::handlebars::TemplateError> for Error {
+        fn from(e: ::handlebars::TemplateError) -> Error {
+            From::from(Box::new(e))
+        }
+    }
 }