Improve Growable.+= and related methods

- Deprecate the varargs version of `Growable.+=`. It is confusing when
  used with tuples (e.g. for `Map` types) and Scala 3.0 will probably
  drop support for infix operators with multiple parameters.
- Add back the specialized two-argument `+=` to `AnyRefMap` which was
  present in 2.12.
- There is no need to deprecate the two-argument versions now. If a
  future Scala version disallows calling them with operator syntax,
  these calls should automatically fall back to the tupled versions.
- Make the tupled `addOne` methods in `AnyRefMap` and `LongMap`
  inlinable. When inlining is enabled the optimizer can elide the
  tupling. Upcoming inliner improvements should also enable this when
  `addOne` is called via the also inlinable `+=` alias.

Author: szeiger

library/src/scala/collection/mutable/AnyRefMap.scala

@@ -285,9 +285,12 @@ class AnyRefMap[K <: AnyRef, V] private[collection] (defaultEntry: K => V, initi
   }
 
   /** Adds a new key/value pair to this map and returns the map. */
-  def addOne(key: K, value: V): this.type = { update(key, value); this }
+  def +=(key: K, value: V): this.type = { update(key, value); this }
 
-  def addOne(kv: (K, V)): this.type = { update(kv._1, kv._2); this }
+  /** Adds a new key/value pair to this map and returns the map. */
+  @inline final def addOne(key: K, value: V): this.type = { update(key, value); this }
+
+  @inline override final def addOne(kv: (K, V)): this.type = { update(kv._1, kv._2); this }
 
   def subtractOne(key: K): this.type = {
     val i = seekEntry(hashOf(key), key)

library/src/scala/collection/mutable/Growable.scala

@@ -36,6 +36,7 @@ trait Growable[-A] extends Clearable {
    *  @param elems the remaining elements to $add.
    *  @return the $coll itself
    */
+  @deprecated("Use `++=` (addAll) instead of varargs `+=`", "2.13.0")
   @`inline` final def += (elem1: A, elem2: A, elems: A*): this.type = this += elem1 += elem2 ++= (elems: IterableOnce[A])
 
   /** ${Add}s all elements produced by an IterableOnce to this $coll.

library/src/scala/collection/mutable/LongMap.scala

@@ -342,7 +342,10 @@ final class LongMap[V] private[collection] (defaultEntry: Long => V, initialBuff
   /** Adds a new key/value pair to this map and returns the map. */
   def +=(key: Long, value: V): this.type = { update(key, value); this }
 
-  override def addOne(kv: (Long, V)): this.type = { update(kv._1, kv._2); this }
+  /** Adds a new key/value pair to this map and returns the map. */
+  @inline final def addOne(key: Long, value: V): this.type = { update(key, value); this }
+
+  @inline override final def addOne(kv: (Long, V)): this.type = { update(kv._1, kv._2); this }
 
   def subtractOne(key: Long): this.type = {
     if (key == -key) {