Prefer FoundationEssentials over Foundation for the Data bridges

[lhoward]

Motivation

The optional Data conveniences in ParserSource.swift and Parsers/Data.swift use public import Foundation, guarded only on canImport(Foundation). Two consequences on non-Darwin (Linux) platforms:

1. Because these are public (re-exported) imports, every downstream consumer of BinaryParsing inherits a link dependency on full Foundation — even modules that never touch Data. The autolink propagates transitively.
2. Full Foundation on Linux pulls in FoundationInternationalization and the ICU data library (_FoundationICU, tens of MB), even though the only symbol used from Foundation here is Data, which is available in FoundationEssentials.

For size-sensitive / embedded-adjacent Linux deployments this drags the entire ICU blob into the link closure of any executable that uses BinaryParsing but not full Foundation. (Found while shrinking a Swift networking daemon for a 64 MB target — BinaryParsing was the sole remaining anchor forcing libFoundation + ICU into an otherwise FoundationEssentials-only binary.)

Change

In both files, prefer FoundationEssentials when available, falling back to full Foundation:

#if !$Embedded && canImport(FoundationEssentials)
public import FoundationEssentials
#elseif !$Embedded && canImport(Foundation)
public import Foundation
#endif

• The canImport(Foundation) guards around the Data extensions are widened to (canImport(FoundationEssentials) || canImport(Foundation)).
• Foundation.Data is de-qualified to Data so it resolves from whichever module was imported.
• $Embedded handling is unchanged.

Effect

No API change — the Data initializers (init(parsingRemainingBytes:), init(parsing:byteCount:)) and the Data: ParserSpanProvider conformance are identical. On Linux this removes libFoundation, libFoundationInternationalization, and the ICU data library from the transitive link closure for consumers that don't otherwise use full Foundation. Verified end-to-end: a dynamically-linked executable depending on BinaryParsing dropped from needing libFoundation + libFoundationInternationalization + _FoundationICU to needing only libFoundationEssentials.

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