Users can now supply additional headers for URL imports using the new `using`
keyword, like this:
```haskell
http://example.com using ./headers
```
... where `./headers` must be a value of type:
```haskell
{ header : Text, value : Text }
```
The argument to `using` must be an import (such as a file, URL, or env import)
and cannot be an inline Dhall expression. There are two reasons why:
* The header expression is resolved, type-checked, and normalized in a separate
phase preceding the import of the corresponding URL so it does not have access
to bound variables in scop
* This restriction greatly simplifies the implementation
Also, headers are automatically forwarded to relative imports, so if you import a
URL like this:
```haskell
http://example.com using ./headers
```
... and that serves a file like:
```haskell
./foo
```
... then Dhall will import `http://example.com/foo` using the same `./headers`.
`trifecta` already depends on `lens`, so using `microlens` does not
actually trim down the dependency tree. Quite the opposite: it adds two
unnecessary dependencies.
