std.uni: Replace parameter storage class in by what it should express…

[carblue]

… (const scope, but dmd treats it as const); see also merged dlang.org PR #2126 or issue #17928

This is a trivial, 'basic' PR in a coming series of PRs dedicated to make std.uni compilable with -dip1000.

Until yesterday's merge of dlang/dlang.org#2126, there was the confusing situatiom, that parameter storage class 'in' was documented in spec as being an equivalent of 'const scope', though dmd treats 'in' as an equivalent of 'const' currently.
Likely, the code author wanted to/should express 'const scope' by using 'in', thus this PR is just a blunt copy-and-paste to sort out the bulk of (-dip1000) compiler errors.
Anyway, I'll check for scope required or not in another pass.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Description
✗	17928	[scope] in is not treated as const scope

[wilzbach]

CC @DmitryOlshansky