Fix the bug with the enums having "Error" variant.

See merge request !9

The "minimal-versions" flag for the Cargo.lock generation contradicts
the expectations of using the currently specified dependencies with the
most novel compiler: the old dependencies may simply be not-in-sync with
the changes in the newer compiler version which those couldn't have even
remotely foreseen during the implementation. Hence, the dependencies
declared in Cargo.toml and synchronised in Cargo.lock should be used.

Also, note that the crate can't declare any backwards compatibility
between the compiler versions, because even the compiler versions break
it, especially among the Rust editions.

This allows the builders and CI to use the up-to-date dependencies.

The file also isn't big; and even if it had been, it is still better to
have it versioned to allow for the build consistency between all the
builders (other repository users, CI, etc), to make sure everyone
actually uses the same dependencies.

It was impossible to have an enum with a variant called "Error", for
example:

enum A {
    Error = 1,
}

Would throw an error due to the ambiguity of using Self::Error in the
TryFrom implementations and a possible reference to A::Error (A is
Self).

This effectively removes the ambiguity by specifically referring to the
TryFrom implementation in the current scope, instead of the enum.

Update `num_traits` doc links

See merge request !8

Use the fully-qualified type name for Result

See merge request !6

Otherwise derived enums failed to build if there's a `Result` type with different number of type arguments in the same scope, e.g:
```
type Result<T> = std::result::Result<T, SomeError>;

 #[derive(Primitive)]
enum Foo {
    A = 1,
}
```

Use core for no_std support

See merge request !5

Implements TryFrom for all the integer types. fixes #4.

Lots of updates

See merge request !4

The quote/syn upgrade in f9b8d46c didn't contain a changelog update.

Upgrade syn and quote

See merge request !3

Switched to the official Rust docker containers for CI testing which
should provide a more consistent test environment. Bumped the minimum
Rust version to 1.32 since 1.15 is super old and not available in the
official containers.

README typo in URL

See merge request !2