Add bleeding edge GCC to CI

[aokellermann]

This PR adds CI testing for the bleeding edge release of GCC (currently GCC 10.1). This is in response to the wanted section in the contributing guidelines. I could not find an issue for this request.

Changes:

• Adds second CircleCI job using Arch Linux docker image to run identical tests as the original.
• Renames original CircleCI job to build_stable to be more descriptive of what it is testing.
• Adds CircleCI workflow to run both jobs concurrently.
• Updates documentation to reflect the addition of this compiler and CI.

Example of successful concurrent jobs:

[coveralls]

Coverage remained the same at 100.0% when pulling 49000cd on aokellermann:CircleCI_GCC_BleedingEdge into c92a696 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!

[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

• Add recent GCC to CI. Add bleeding edge GCC to CI #2136

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