Added TomCrypt as a Swift Package. Lots taken from libtom/libtomcrypt#575

Author: DavidSouthgate

.gitignore

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+**/xcuserdata/*

.gitmodules

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+[submodule "vendor"]
+	path = vendor
+	url = git@github.com:libtom/libtomcrypt.git

.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

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+module TomCrypt [extern_c] {
+    header "tomcrypt.h"
+    export *
+}

Headers/module.modulemap

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+#include "../vendor/src/headers/tomcrypt.h"

Headers/tomcrypt.h

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+{
+  "object": {
+    "pins": [
+      {
+        "package": "TomMath",
+        "repositoryURL": "https://github.com/DavidSouthgate/TomMathSPM.git",
+        "state": {
+          "branch": "main",
+          "revision": "d14f446ea2f0de6cf9d907f408a50133ed53bdd9",
+          "version": null
+        }
+      }
+    ]
+  },
+  "version": 1
+}

Package.resolved

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+// swift-tools-version:5.0
+import PackageDescription
+
+let package = Package(
+    name: "TomCrypt",
+    platforms: [
+        .macOS(.v10_10), .iOS(.v9), .tvOS(.v9), .watchOS(.v4)
+    ],
+    products: [
+        .library(
+            name: "TomCrypt",
+            targets: [ "TomCrypt" ])
+    ],
+    dependencies: [
+        .package(url: "https://github.com/DavidSouthgate/TomMathSPM.git", .branchItem("main"))
+    ],
+    targets: [
+        .target(name: "TomCrypt",
+                dependencies: [
+                    .product(name: "TomMath", package: "TomMathSPM")
+                ],
+                path: ".",
+                sources: [
+                    "vendor/src"
+                ],
+                publicHeadersPath: "Headers",
+                cSettings: [
+                    .headerSearchPath("vendor/src/headers"),
+                    .define("USE_LTM"),
+                    .define("LTM_DESC"),
+                    .define("LTC_NO_TEST"),
+                    .unsafeFlags(["-flto=thin"])  // for Dead Code Elimination
+                ]),
+        .testTarget(name: "TomCryptTests",
+                   dependencies: [
+                        "TomCrypt"
+                   ],
+                    path: "Tests")
+    ],
+    cLanguageStandard: .gnu11,
+    cxxLanguageStandard: .gnucxx14
+)