Improve string concatentation & string templates code generation

Reuse StringBuilder instances for nested subexpressions.
(NB StringBuilder instance for string template with a string
concatenation inside an expression entry, such as `"${"a" + "b"}"`,
will not be reused, although that doesn't seem to be a real-life issue).

 #KT-18558 Fixed Target versions 1.1.4
 #KT-13682 Fixed Target versions 1.1.4

Join adjacent strings literals, escaped strings, and constant values
(in a language version that supports const val inlining).
Use StringBuilder#append(char) for single-character constants
(e.g., " " in "$a $b").

 #KT-17280 Fixed Target versions 1.1.4
 #KT-15235 Fixed Target versions 1.1.4

compiler/testData/codegen/bytecodeText/stringOperations/kt15235.kt

@@ -0,0 +1,4 @@
+val c = 1
+fun foo() = "a\"($c)\"d"
+
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append

compiler/testData/codegen/bytecodeText/stringOperations/nestedConcat.kt

@@ -0,0 +1,15 @@
+fun test1(s1: String, s2: String, s3: String) =
+        (s1 + s2) + s3
+
+fun test2(s1: String, s2: String, s3: String) =
+        s1 + (s2 + s3)
+
+fun test3(s1: String, s2: String, s3: String, s4: String) =
+        ((s1 + s2) + ((s3 + s4)))
+
+fun test4(s1: String, s2: String, s3: String) =
+        "s1: $s1; " +
+        "s2: $s2; " +
+        "s3: $s3"
+
+// 4 NEW java/lang/StringBuilder