Massive binary bloat in std::print

Consider the following trivial program (`std-print.cc`):

```c++
#include

int main() {
std::print("{}\n", 42);
}
```

Compiling it with libc++ and Apple clang version 17.0.0 (clang-1700.3.19.1) on macOS results in a ~300kB object file:

```
% c++ -c -std=c++23 std-print.cc
% ls -l std-print.o
-rw-r--r-- 1 viz staff 297384 Oct 11 07:07 std-print.o
```

For comparison, an equivalent program using [{fmt}](https://github.com/fmtlib/fmt) (`fmt-print.cc`)

```c++
#include

int main() {
fmt::print("{}\n", 42);
}
```

compiles to less than 1kB.
```
% c++ -c -std=c++23 fmt-print.cc -I include
% ls -l fmt-print.o
-rw-r--r-- 1 viz staff 968 Oct 11 07:08 fmt-print.o
```

So the implementation of `std::print` in libc++ generates ~300 times (!) more binary code and is ~5.5 times slower to compile. This is likely due to unnecessary templatization and defining in the header of `vprint_unicode`:

https://github.com/llvm/llvm-project/blob/7eee67202378932d03331ad04e7d07ed4d988381/libcxx/include/print#L387-L390

`std::print` was explicitly designed such that `vprint*` and other similar functions were defined in library source files and not headers specifically to avoid such bloat and slow compile times.