Integer fast-path for hashCode calculation

[JakeWharton]

An integer is its own hashCode, and the function is documented to simply return the input on the JVM. For JS and native the behavior is the same: https://github.com/JetBrains/kotlin/blob/1.9.0/kotlin-native/runtime/src/main/kotlin/kotlin/Primitives.kt#L1353.

Saves at least 3 bytes per property within the function bytecode, not counting any effects on things like the constant pool (for JVM).

Before:

public int hashCode();
  Code:
     0: aload_0
     1: getfield      #23                 // Field int:I
     4: invokestatic  #50                 // Method java/lang/Integer.hashCode:(I)I
     7: istore_1
     8: iload_1
     9: bipush        31
    11: imul
     ⋮
    46: ireturn

After:

public int hashCode();
  Code:
     0: aload_0
     1: getfield      #23                 // Field int:I
     4: istore_1
     5: iload_1
     6: bipush        31
     8: imul
     ⋮
    43: ireturn

Future work (will add to issue):

• Support UInt by emitting a call to toInt() which should basically be a no-op (and eliminated completely on the JVM at least).
• Eliminate wasteful store/load instructions between operations.

Refs #204