[isoltest] Add account builtin.

Co-authored-by: Alex Beregszaszi <alex@rtfs.hu>

test/libsolidity/semanticTests/isoltestTesting/account.sol

@@ -0,0 +1,48 @@
+contract AccountBuiltinTest {
+    function who_am_i() public returns (address result) {
+        result = msg.sender;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// constructor()
+// account: 0 -> 0x1212121212121212121212121212120000000012
+// who_am_i() -> 0x1212121212121212121212121212120000000012
+// balance: 0x1212121212121212121212121212120000000012 -> 1267650600228229401496703205376
+// account: 1 -> 0x1212121212121212121212121212120000001012
+// who_am_i() -> 0x1212121212121212121212121212120000001012
+// balance: 0x1212121212121212121212121212120000001012 -> 1267650600228229401496703205376
+// account: 2 -> 0x1212121212121212121212121212120000002012
+// who_am_i() -> 0x1212121212121212121212121212120000002012
+// balance: 0x1212121212121212121212121212120000002012 -> 1267650600228229401496703205376
+// account: 3 -> 0x1212121212121212121212121212120000003012
+// who_am_i() -> 0x1212121212121212121212121212120000003012
+// balance: 0x1212121212121212121212121212120000003012 -> 1267650600228229401496703205376
+// account: 4 -> 0x1212121212121212121212121212120000004012
+// who_am_i() -> 0x1212121212121212121212121212120000004012
+// balance: 0x1212121212121212121212121212120000004012 -> 1267650600228229401496703205376
+// account: 5 -> 0x1212121212121212121212121212120000005012
+// who_am_i() -> 0x1212121212121212121212121212120000005012
+// balance: 0x1212121212121212121212121212120000005012 -> 1267650600228229401496703205376
+// account: 6 -> 0x1212121212121212121212121212120000006012
+// who_am_i() -> 0x1212121212121212121212121212120000006012
+// balance: 0x1212121212121212121212121212120000006012 -> 1267650600228229401496703205376
+// account: 7 -> 0x1212121212121212121212121212120000007012
+// who_am_i() -> 0x1212121212121212121212121212120000007012
+// balance: 0x1212121212121212121212121212120000007012 -> 1267650600228229401496703205376
+// account: 8 -> 0x1212121212121212121212121212120000008012
+// who_am_i() -> 0x1212121212121212121212121212120000008012
+// balance: 0x1212121212121212121212121212120000008012 -> 1267650600228229401496703205376
+// account: 9 -> 0x1212121212121212121212121212120000009012
+// who_am_i() -> 0x1212121212121212121212121212120000009012
+// balance: 0x1212121212121212121212121212120000009012 -> 1267650600228229401496703205376
+// account: 10 -> 0x121212121212121212121212121212000000a012
+// who_am_i() -> 0x121212121212121212121212121212000000a012
+// balance: 0x121212121212121212121212121212000000a012 -> 0
+// account: 11 -> 0x121212121212121212121212121212000000b012
+// who_am_i() -> 0x121212121212121212121212121212000000b012
+// balance: 0x121212121212121212121212121212000000b012 -> 0
+// account: 12 -> 0x121212121212121212121212121212000000c012
+// who_am_i() -> 0x121212121212121212121212121212000000c012
+// balance: 0x121212121212121212121212121212000000c012 -> 0