Bump im from 10.2.0 to 12.3.4

[dependabot-preview]

Bumps im from 10.2.0 to 12.3.4.

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.

[12.3.4] - 2019-04-08

Changed

• Clone constraints have been further relaxed on maps and sets, so that you
  can now lookup and iterate over them without requiring a Clone constraint
  (though you do still need Clone to actually insert data into them to lookup
  or iterate over). (#81)

Fixed

• Enforces the latest bugfix release of sized-chunks. (#78)
• Another edge case bugfix to Vector's size table handling. (#79)

[12.3.3] - 2019-03-11

Fixed

• A number of issues were fixed where Vector's size table would get out of
  sync with the node structure if exercised too much and cause erroneous
  behaviour. (#72, #74)
• Comprehensive generative tests were added to test all data structures through
  more unexpected code paths.

[12.3.2] - 2019-03-05

Changed

• Clone constraints on all data structures, as well as relevant constraints on
  maps and sets, have been relaxed where possible, so that you can now construct
  empty instances and call most query methods without requiring values implement
  Clone etc. (#63)

Fixed

• Constructing an empty Vector will not allocate any heap memory, instead
  deferring allocation until you perform an operation that would increase its
  length. (#65)
• Some bugs arising when using Vector::append repeatedly were fixed. (#67,
  #70)

[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

... (truncated)

Commits

• 12ecec9 (cargo-release) version 12.3.4
• 4dd6082 Remove Clone constraints on map/set ops where it can be done without rewrit...
• ff35b94 Changelog for #79.
• 157a264 Require sized-chunks 0.1.2 to avoid Fix drain_from_back bodil/sized-chunks#1.
• 350f5ac Merge pull request #79 from krobelus/issue-77
• 1b19e53 Merge pull request #82 from pzmarzly/patch-1
• f1c55b0 Remove unnecessary clippy.toml field
• 617c782 RRB tree: use size table after pushing a sparse node to the left
• 35712f1 (cargo-release) version 12.3.3
• 779f742 Comprehensive tests validate the reality of the size table at each step.
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #30.