Delegate null-check-then-equals to Objects.equals

[retronym]

Motivation is to remove avoidable synthetic branches/instructions
for null data that are intepreted as uncovered code by JaCoCo etc.

Bytecode is also reduced.

Note that if either operand is potential Number or Character,
the backend delegates to BoxesRuntime.equals which handles Scala
semantic edge cases for boxed primitives. This PR does not change
that at all.

Before:

  public boolean test1(java.lang.String, java.lang.String);
    descriptor: (Ljava/lang/String;Ljava/lang/String;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=4, args_size=3
         0: aload_1
         1: aload_2
         2: astore_3
         3: dup
         4: ifnonnull     15
         7: pop
         8: aload_3
         9: ifnull        22
        12: goto          26
        15: aload_3
        16: invokevirtual #20                 // Method java/lang/Object.equals:(Ljava/lang/Object;)Z
        19: ifeq          26
        22: iconst_1
        23: goto          27
        26: iconst_0
        27: ireturn

After

  public boolean test1(java.lang.String, java.lang.String);
    descriptor: (Ljava/lang/String;Ljava/lang/String;)Z
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=3
         0: aload_1
         1: aload_2
         2: invokestatic  #22                 // Method java/util/Objects.equals:(Ljava/lang/Object;Ljava/lang/Object;)Z
         5: ifeq          12
         8: iconst_1
         9: goto          13
        12: iconst_0
        13: ireturn