feat(Order): Trimmed length of a RelSeries

[Raph-DG]

In this PR, we define the trimmed length of a relseries rs : RelSeries (· ≤ ·) to be the cardinality of the underlying function rs.toFun of rs minus 1. This models the number of < relations occuring in rs. We develop some basic API for this notion, with the main intended application being a proof that module length is additive.

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[github-actions]

PR summary 07d6cda80f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Order.TrimmedLength (new file)	579

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Declarations diff

+ RelSeries.length_eq_trimmedLength_iff
+ RelSeries.trim
+ RelSeries.trimmedLength
+ RelSeries.trimmedLength_eraseLast_le
+ RelSeries.trimmedLength_eraseLast_of_eq
+ RelSeries.trimmedLength_eraseLast_of_lt
+ RelSeries.trimmedLength_exists_le
+ RelSeries.trimmedLength_le_length
+ instance (rs : RelSeries (α := α) (· ≤ ·)) :
+ rel_of_le

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).