Don't use `fmt.Sprintf("%s", x)` unnecessarily

In many instances, there are easier and more efficient ways of getting
a value's string representation. Whenever a value's underlying type is
a string already, or the type has a `String` method, they should be
used directly.

Given the following shared definitions

```
type T1 string
type T2 int

func (T2) String() string { return "Hello, world" }

var x string
var y T1
var z T2
```

we can simplify the following

```
fmt.Sprintf("%s", x)
fmt.Sprintf("%s", y)
fmt.Sprintf("%s", z)
```

to

```
x
string(y)
z.String()
```
