[Merged by Bors] - feat: make Sub ℝ≥0 and Sub ℝ≥0∞ computable

[eric-wieser]

This comes for free by relaxing some unnecessarily-strong typeclass requirements.

(as a reminder, LinearOrder NNReal is noncomputable as it contains noncomputable Decidable instances)

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[github-actions]

PR summary 871423b0d7

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[Ruben-VandeVelde]

Seems harmless assuming it builds.

maintainer delegate

[github-actions]

🚀 Pull request has been placed on the maintainer queue by Ruben-VandeVelde.

[eric-wieser]

!bench

[leanprover-bot]

Here are the benchmark results for commit 871423b.
There were no significant changes against commit a1ad6eb.

[github-actions]

File	Instructions	%
build	-11.239⬝10⁹	(+0.00%)
CI run

[RemyDegenne]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• Build
• Lint style