-preview=in: Replace 'in ref' with 'const ref'

Allow the code to compile with '-preview=in'.

Merge pull request dlang-community#154 from dlang-community/more-tree…

…map-fixes

Fix a few problems with TreeMap.getOrAdd
merged-on-behalf-of: Brian Schott <Hackerpilot@users.noreply.github.com>

Merge pull request dlang-community#153 from dlang-community/ttree-fix

Fix more dumb issues with tree ranges

Merge pull request dlang-community#152 from dlang-community/tree-tweaks

Make some enhancements to the treemap structure, and to the T-Tree do…
merged-on-behalf-of: Richard Andrew Cattermole <alphaglosined@gmail.com>

Merge pull request dlang-community#150 from dlang-community/fix_ref_r…

…eturns

Fix dlang-community#148
merged-on-behalf-of: Brian Schott <Hackerpilot@users.noreply.github.com>

Merge pull request dlang-community#144 from dlang-community/issue-143

fix dlang-community#143 - Still issues in unrolled list
merged-on-behalf-of: Basile-z <Basile-z@users.noreply.github.com>

Merge pull request dlang-community#145 from dlang-community/fix_insert

Fix use-after-free when an insert causes a rehash

Merge pull request dlang-community#141 from dlang-community/stdx-allo…

…cs+Meson

Fix CI failure starting from DMD 2.085

Merge pull request dlang-community#135 from dlang-community/optimize_…

…hashToIndex

Optimize hash to index using LDC intrinsics

Merge pull request dlang-community#129 from Hackerpilot/hash-nonsense

Improve hash code to array index translation