Bump im from 10.2.0 to 12.3.1

[dependabot-preview]

Bumps im from 10.2.0 to 12.3.1.

Release notes

Sourced from im's releases.

12.3.1

Changed

• Unsafe chunks have been separated out into the sized-chunks crate, which is now a dependency of im.

Changelog

Sourced from im's changelog.

[12.3.1] - 2019-02-19

Changed

• Unsafe chunks have been separated out into the sized-chunks crate, which is
  now a dependency of im.

[12.3.0] - 2019-01-15

Added

• singleton methods have been deprecated and renamed to unit.
• Vector::chunks and Vector::chunks_mut have been deprecated and renamed to
  leaves and leaves_mut to avoid confusion with Vec::chunks. (#50)

Fixed

• Fixed an issue where the HashMap draining iterator might access uninitialised memory leading to
  undefined behaviour. (#60)
• Fixed multiple issues in Vector::split_off and Vector::append that would cause lookup errors
  and unexpectedly unbalanced trees. (#55).

[12.2.0] - 2018-10-12

Added

• OrdMap and OrdSet now have a range() method which makes an iterator over
  a bounded subset of the values. The improved iterator implementation is also
  considerably more efficient than the previous (about an order of magnitude
  faster for nontrivial data sets). iter() has been updated to take advantage
  of this, and is now just an alias for range(..). (#27)
• FocusMut now has an unmut method to turn it into an immutable Focus,
  releasing its exclusive hold on the underlying Vector.
• Focus now implements Clone.

[12.1.0] - 2018-09-25

Added

• Maps and sets now have the clear method just like Vector. (#46)

Changed

• Single chunk Vectors are no longer allocated directly on the stack, meaning
  that they're now comparable in performance to std::vec::Vec rather than
  slightly faster, but they also won't eat up your stack space quite as quickly,
  and they'll clone without copying and share structure with clones as you'd
  expect.

[12.0.0] - 2018-08-30

Starting with this release, the arc flag is gone, in favour of publishing im
as two separate crates: im (using Arc) and im-rc (using Rc). They're

... (truncated)

Commits

• 2295bc9 (cargo-release) version 12.3.1
• e525738 Run prepare-release before make release commit.
• 1fa3b79 Move the unsafe chunks into the sized-chunks crate.
• 8373b71 Use mem::zeroed to allocate unsafe chunks.
• ef803bf Run rustup update before each build because the Rust installation is cached...
• e117ecf Don't use deprecated singleton and chunk methods in tests.
• 2801bb7 Cache rustup installation, and run clippy on stable.
• cd93779 Update proptest requirement from 0.8 to 0.9
• 9e18e88 Sync rc/Cargo.toml after release.
• c901d48 (cargo-release) version 12.3.0
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #24.