message: Avoid allocation of default JPEG tables
diff: use std::convert::TryFrom;
  use std::io::{self, Write};
  +use std::borrow::Cow;
  use num_iter::range_step;
  @@ -339,10 +340,10 @@ pub struct JpegEncoder<'a, W: 'a> {
  components: Vec<Component>,
  tables: Vec<[u8; 64]>,
  - luma_dctable: Box<[(u8, u16); 256]>,
  - luma_actable: Box<[(u8, u16); 256]>,
  - chroma_dctable: Box<[(u8, u16); 256]>,
  - chroma_actable: Box<[(u8, u16); 256]>,
  + luma_dctable: Cow<'static, [(u8, u16); 256]>,
  + luma_actable: Cow<'static, [(u8, u16); 256]>,
  + chroma_dctable: Cow<'static, [(u8, u16); 256]>,
  + chroma_actable: Cow<'static, [(u8, u16); 256]>,
  pixel_density: PixelDensity,
  }
  @@ -421,10 +422,10 @@ impl<'a, W: Write> JpegEncoder<'a, W> {
  components,
  tables,
  - luma_dctable: Box::new(STD_LUMA_DC_HUFF_LUT),
  - luma_actable: Box::new(STD_LUMA_AC_HUFF_LUT),
  - chroma_dctable: Box::new(STD_CHROMA_DC_HUFF_LUT),
  - chroma_actable: Box::new(STD_CHROMA_AC_HUFF_LUT),
  + luma_dctable: Cow::Borrowed(&STD_LUMA_DC_HUFF_LUT),
  + luma_actable: Cow::Borrowed(&STD_LUMA_AC_HUFF_LUT),
  + chroma_dctable: Cow::Borrowed(&STD_CHROMA_DC_HUFF_LUT),
  + chroma_actable: Cow::Borrowed(&STD_CHROMA_AC_HUFF_LUT),
  pixel_density: PixelDensity::default(),
  }
  