refactor: make all extensions internal

We don't want to expose these extensions, even though they might be
useful for other projects, because we want to have lean public APIs
which are easier to maintain.

Sources/ImmutableXCore/Extensions/Array+Extensions.swift

@@ -1,9 +1,9 @@
 import Foundation
 
 // https://github.com/krzyzanowskim/CryptoSwift/blob/main/Sources/CryptoSwift/Array%2BExtension.swift#L34
-public extension Array where Element == UInt8 {
+extension Array where Element == UInt8 {
     @inlinable
-    internal init(reserveCapacity: Int) {
+    init(reserveCapacity: Int) {
         self = [Element]()
         self.reserveCapacity(reserveCapacity)
     }

Sources/ImmutableXCore/Extensions/BigInt+Extensions.swift

@@ -1,7 +1,7 @@
 import BigInt
 import Foundation
 
-public extension BigInt {
+extension BigInt {
     var isEven: Bool {
         magnitude[bitAt: 0] == false
     }

Sources/ImmutableXCore/Extensions/Data+Extensions.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public extension Data {
+extension Data {
     init(hex: String) {
         self.init([UInt8](hex: hex))
     }

Sources/ImmutableXCore/Extensions/Double+Extensions.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public extension Double {
+extension Double {
     /// 64-bit floating-point number (double), printed in the style of %e if the exponent is less than –4 or greater than or equal to the precision, in the style of %f otherwise.
     var asString: String {
         // https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Strings/Articles/formatSpecifiers.html

Sources/ImmutableXCore/Extensions/Int+Extensions.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public extension Int {
+extension Int {
     var asHexString: String {
         String(format: "%02x", self)
     }

Sources/ImmutableXCore/Extensions/String+Extensions.swift

@@ -1,6 +1,6 @@
 import Foundation
 
-public extension String {
+extension String {
     /// Drops "0x" prefix if exists
     var dropHexPrefix: String {
         hasPrefix("0x") ? String(dropFirst(2)) : self