Convert to edition 2018

Notably, This enables NLL even on older compilers such as 1.34.

src/encoder/colortype.rs

@@ -1,4 +1,4 @@
-use tags::{PhotometricInterpretation, SampleFormat};
+use crate::tags::{PhotometricInterpretation, SampleFormat};
 
 /// Trait for different colortypes that can be encoded.
 pub trait ColorType {

src/encoder/mod.rs

@@ -3,9 +3,9 @@ use std::convert::TryFrom;
 use std::io::{Seek, Write};
 use std::{cmp, io, mem};
 
-use bytecast;
-use error::{TiffError, TiffFormatError, TiffResult};
-use tags::{self, ResolutionUnit, Tag, Type};
+use crate::bytecast;
+use crate::error::{TiffError, TiffFormatError, TiffResult};
+use crate::tags::{self, ResolutionUnit, Tag, Type};
 
 pub mod colortype;
 mod writer;

src/encoder/writer.rs

@@ -1,4 +1,4 @@
-use error::TiffResult;
+use crate::error::TiffResult;
 use std::io::{self, Seek, SeekFrom, Write};
 
 pub fn write_tiff_header<W: Write>(writer: &mut TiffWriter<W>) -> TiffResult<()> {