WASM: Implement wasm-native strings part 1

There are several changes here but they all required for at least one test to pass.
- Implemented String class and several utility functions using built-in CharArray
- Added new constant memory segment to hold string literals and required funcs to work with them
- Added very crude mostly incorrect rudimentary ability to pass strings back to javascript

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/WasmExpressionBuilder.kt

@@ -145,5 +145,9 @@ abstract class WasmExpressionBuilder {
     fun buildRttCanon(decl: WasmSymbol<WasmTypeDeclaration>) {
         buildInstr(WasmOp.RTT_CANON, WasmImmediate.TypeIdx(decl))
     }
+
+    fun buildDrop() {
+        buildInstr(WasmOp.DROP)
+    }
 }