Replace all Foundation imports with FoundationEssentials (#363)

### Motivation:

FoundationEssentials produces smaller binaries on most platforms.

### Modifications:

Where FoundationEssentials is available, import that.

### Result:

Improved binary size

Author: Lukasa

Benchmarks/Benchmarks/Benchmarks.swift

@@ -13,9 +13,14 @@
 //===----------------------------------------------------------------------===//
 import Benchmark
 import Crypto
-import Foundation
 import _CryptoExtras
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
+import Foundation
+#endif
+
 let benchmarks = {
     let defaultMetrics: [BenchmarkMetric] = [.mallocCountTotal, .cpuTotal]
 

Sources/Crypto/AEADs/AES/GCM/AES-GCM.swift

@@ -25,8 +25,12 @@ typealias AESGCMImpl = OpenSSLAESGCMImpl
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 public import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
 #endif
+#endif
 
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
 extension AES {

Sources/Crypto/AEADs/AES/GCM/BoringSSL/AES-GCM_boring.swift

@@ -16,7 +16,11 @@
 #else
 @_implementationOnly import CCryptoBoringSSL
 import CryptoBoringWrapper
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
 enum OpenSSLAESGCMImpl {

Sources/Crypto/AEADs/ChachaPoly/BoringSSL/ChaChaPoly_boring.swift

@@ -17,7 +17,11 @@
 @_implementationOnly import CCryptoBoringSSL
 @_implementationOnly import CCryptoBoringSSLShims
 import CryptoBoringWrapper
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
+#endif
 
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
 extension BoringSSLAEAD {

Sources/Crypto/AEADs/ChachaPoly/ChaChaPoly.swift

@@ -25,8 +25,12 @@ typealias ChaChaPolyImpl = OpenSSLChaChaPolyImpl
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 public import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
 #endif
+#endif
 
 
 /// An implementation of the ChaCha20-Poly1305 cipher.

Sources/Crypto/AEADs/Cipher.swift

@@ -18,8 +18,12 @@
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
 #endif
+#endif
 
 
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)

Sources/Crypto/AEADs/Nonces.swift

@@ -17,8 +17,12 @@
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 public import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
 #endif
+#endif
 // MARK: - Generated file, do NOT edit
 // any edits of this file WILL be overwritten and thus discarded
 // see section `gyb` in `README` for details.

Sources/Crypto/AEADs/Nonces.swift.gyb

@@ -17,8 +17,12 @@
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 public import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+public import FoundationEssentials
+#else
 public import Foundation
 #endif
+#endif
 // MARK: - Generated file, do NOT edit
 // any edits of this file WILL be overwritten and thus discarded
 // see section `gyb` in `README` for details.

Sources/Crypto/ASN1/ASN1.swift

@@ -18,8 +18,12 @@
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
 #endif
+#endif
 
 // This module implements "just enough" ASN.1. Specifically, we implement exactly enough ASN.1 DER parsing to handle
 // the following use-cases:

Sources/Crypto/ASN1/Basic ASN1 Types/ASN1Any.swift

@@ -18,8 +18,12 @@
 #if CRYPTOKIT_NO_ACCESS_TO_FOUNDATION
 import SwiftSystem
 #else
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
 import Foundation
 #endif
+#endif
 
 @available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, macCatalyst 13, visionOS 1.0, *)
 extension ASN1 {