Fix quadratic destruction complexity

[nickaein]

Fixes #1837 along with minor cleanups.

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[coveralls]

Coverage remained the same at 100.0% when pulling efa13c6 on nickaein:fix-quadratic-destruction into be61ad1 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks everybody!!!

[nlohmann]

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🔖 Release item

This issue/PR will be part of the next release of the library. This template helps preparing the release notes.

Type

• ✨ New Feature
• 🐛 Bug Fix
• ⚡️ Improvement
• 🔨 Further Change
• 🔥 Deprecated function

Description

• Removed reserve() calls from the destructor which could lead to quadratic complexity. Quadratic destruction complexity introduced in #1436 #1837 Fix quadratic destruction complexity #1838

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