Overhaul `RefPtr` and `String`

- `RefPtr` no longer tries to have distinct cases for interal-vs-external reference counts. Instead we always require an internal reference count.

  - Types the used `RefPtr` but weren't `RefObject` were made to inherit `RefObject`

  - The `ReferenceCounted` base class was removed, so that only `RefObject` remains

  - Implicit conversion from `RefPtr<T>` to `T*` added
    - This created some complicates for other types that relied on implicit conversions, so this isn't a net cleanup right now

- The main type that got messed up by the above was `String`, which previously held a `RefPtr<char, ...>`. This change thus *also* includes a major overhaul of `String`:

  - `String` now holds all its data via indirection, using a `StringRepresentation` that is a `RefObject`. This object holds a length, capacity, and directly stores the character data in its allocation. This means that `sizeof(String)==sizeof(void*)`

  - It is now possible to directly mutate a `String` by appending to its representation (we just need to ensure it has a reference count of `1`, possibly by cloning it). This means that `StringBuilder` is now basically just an idomatic use of `String`

  - A couple operations that just return sub-ranges of a `String` now return `StringSlice` to avoid allocation when it isn't needed. This required more work.

  - Indices into strings changed from `int` to `UInt` (which is pointer-sized). This had a bunch of follow-on changes because the value `-1` sometimes needs to be special-cased in code that uses indices. Further cleanups are probably needed here.

2 files changed, 4 insertions, 4 deletions

diff --git a/tools/slang-test/main.cpp b/tools/slang-test/main.cpp
index 7a0cac3ce..2bca2f78d 100644
--- a/tools/slang-test/main.cpp
+++ b/tools/slang-test/main.cpp
@@ -548,7 +548,7 @@ OSError spawnAndWait(String	testPath, OSProcessSpawner& spawner)
 {
     if( options.shouldBeVerbose )
     {
-        fprintf(stderr, "%s\n", spawner.commandLine_.Buffer());
+        fprintf(stderr, "%s\n", spawner.commandLine_.ToString().begin());
     }
 
     OSError err = spawner.spawnAndWaitForCompletion();
@@ -1149,7 +1149,7 @@ void handleTestResult(
                 break;
             }
 
-            printf("%s test: '%S'\n", resultString, testName.ToWString());
+            printf("%s test: '%S'\n", resultString, testName.ToWString().begin());
         }
         break;
 
diff --git a/tools/slang-test/os.cpp b/tools/slang-test/os.cpp
index 63db4f9ad..40fd8afff 100644
--- a/tools/slang-test/os.cpp
+++ b/tools/slang-test/os.cpp
@@ -314,8 +314,8 @@ OSError OSProcessSpawner::spawnAndWaitForCompletion()
 
     // `CreateProcess` requires write access to this, for some reason...
     BOOL success = CreateProcessW(
-        isExecutablePath_ ? executableName_.ToWString() : nullptr,
-        (LPWSTR)commandLine_.ToString().ToWString(),
+        isExecutablePath_ ? executableName_.ToWString().begin() : nullptr,
+        (LPWSTR)commandLine_.ToString().ToWString().begin(),
         nullptr,
         nullptr,
         true,