Template inheritance (#9)

* Move all context variables into HBMustacheContext

* Add support for reading inherited sections

* Render inherited tokens

* Test inheritance spec, fix two minor issues

* fix warning

* swift format

Sources/HummingbirdMustache/Context.swift

@@ -0,0 +1,57 @@
+struct HBMustacheContext {
+    let stack: [Any]
+    let sequenceContext: HBMustacheSequenceContext?
+    let indentation: String?
+    let inherited: [String: HBMustacheTemplate]?
+
+    init(_ object: Any) {
+        self.stack = [object]
+        self.sequenceContext = nil
+        self.indentation = nil
+        self.inherited = nil
+    }
+
+    private init(
+        stack: [Any],
+        sequenceContext: HBMustacheSequenceContext?,
+        indentation: String?,
+        inherited: [String: HBMustacheTemplate]?
+    ) {
+        self.stack = stack
+        self.sequenceContext = sequenceContext
+        self.indentation = indentation
+        self.inherited = inherited
+    }
+
+    func withObject(_ object: Any) -> HBMustacheContext {
+        var stack = self.stack
+        stack.append(object)
+        return .init(stack: stack, sequenceContext: nil, indentation: self.indentation, inherited: self.inherited)
+    }
+
+    func withPartial(indented: String?, inheriting: [String: HBMustacheTemplate]?) -> HBMustacheContext {
+        let indentation: String?
+        if let indented = indented {
+            indentation = (self.indentation ?? "") + indented
+        } else {
+            indentation = self.indentation
+        }
+        let inherits: [String: HBMustacheTemplate]?
+        if let inheriting = inheriting {
+            if let originalInherits = self.inherited {
+                inherits = originalInherits.merging(inheriting) { value, _ in value }
+            } else {
+                inherits = inheriting
+            }
+        } else {
+            inherits = self.inherited
+        }
+        return .init(stack: self.stack, sequenceContext: nil, indentation: indentation, inherited: inherits)
+    }
+
+    func withSequence(_ object: Any, sequenceContext: HBMustacheSequenceContext) -> HBMustacheContext {
+        var stack = self.stack
+        stack.append(object)
+        return .init(stack: stack, sequenceContext: sequenceContext, indentation: self.indentation, inherited: self.inherited)
+    }
+}