Dependencies of this crate now use `const_fn` which requires 1.46.0.

Signed-off-by: Nathaniel McCallum <[email protected]>

```
warning: this seems like a manual implementation of the non-exhaustive pattern
  --> src/error.rs:11:1
   |
11 |   pub enum Error {
   |   ^-------------
   |   |
   |  _help: add the attribute: `#[non_exhaustive] pub enum Error`
   | |
12 | |     ParseError {
13 | |         msg: String,
14 | |         line: usize,
...  |
45 | |     __NonExhaustive,
46 | | }
   | |_^
   |
   = note: `#[warn(clippy::manual_non_exhaustive)]` on by default
help: remove this variant
  --> src/error.rs:45:5
   |
45 |     __NonExhaustive,
   |     ^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_non_exhaustive

warning: 1 warning emitted
```

Signed-off-by: Nathaniel McCallum <[email protected]>

```
error: this expression borrows a reference (`&std::string::String`) that is immediately dereferenced by the compiler
   --> src/error.rs:114:44
    |
114 |             Error::GenericError { msg } => &msg,
    |                                            ^^^^ help: change this to: `msg`
    |
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
    = help: for further information visit rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: aborting due to previous error
```

Signed-off-by: Nathaniel McCallum <[email protected]>

This implements an internal `Value` type which functions like
`serde_json::Value`. Note that because this commit changes the `Error`
type, this is an API-breaking change.

Resolves: bheisler#1

Signed-off-by: Nathaniel McCallum <[email protected]>