Remove a few unneeded mutatings

Author: mattmassicotte

Sources/TextFormation/NewlineWithinPairFilter.swift

@@ -13,7 +13,7 @@ public struct NewlineWithinPairFilter<Interface: TextSystemInterface> {
 }
 
 extension NewlineWithinPairFilter: Filter {
-	public mutating func processMutation(_ mutation: TextMutation<Interface>) throws -> Interface.Output? {
+	public func processMutation(_ mutation: TextMutation<Interface>) throws -> Interface.Output? {
 		let interface = mutation.interface
 		let pos = mutation.range.lowerBound
 

Sources/TextFormation/OpenPairReplacementFilter.swift

@@ -11,7 +11,7 @@ public struct OpenPairReplacementFilter<Interface: TextSystemInterface> {
 }
 
 extension OpenPairReplacementFilter: Filter {
-	public mutating func processMutation(_ mutation: TextMutation<Interface>) throws -> Interface.Output? {
+	public func processMutation(_ mutation: TextMutation<Interface>) throws -> Interface.Output? {
 		let interface = mutation.interface
 		let start = mutation.range.lowerBound
 		let end = mutation.range.upperBound

Sources/TextFormation/StandardOpenPairFilter.swift

@@ -7,8 +7,8 @@ public struct StandardOpenPairFilter<Interface: TextSystemInterface> {
 	private let skip: SkipFilter<Interface>
 	private var closeWhitespace: LineLeadingWhitespaceFilter<Interface>?
 	private var closePair: ClosePairFilter<Interface>
-	private var newlinePair: NewlineWithinPairFilter<Interface>
-	private var openPairReplacement: OpenPairReplacementFilter<Interface>
+	private let newlinePair: NewlineWithinPairFilter<Interface>
+	private let openPairReplacement: OpenPairReplacementFilter<Interface>
 	private let deleteClose: DeleteCloseFilter<Interface>
 
 	public init(open: String, close: String) {