Remove very expensive unwind_protect() in string proxy assignment and push back

[DavisVaughan]

I have determined that the unwind_protect()s being utilized in the string proxy assignment operator and the string push_back() operator are doing much more damage than they are worth.

• No other proxy assignment operator or push_back() operator for other vector types is performing this protection. Not even for lists, which has near identical implementations between SET_VECTOR_ELT() and SET_STRING_ELT().
• Outside of SET_STRING_ELT() itself, the only thing to look at for "weirdness" would be the implicit conversion operator from r_string to SEXP. But that's a no-op, it just returns the underlying data_, so there is no need for unwind_protect() for that.

Because of the above 2 points, I've determined that removing the unwind_protect()s in the string impl would not make it any less safe than using any other vector type, which seem to have been working fine so far.

[[cpp11::register]] cpp11::writable::strings string_proxy_assignment_() {
  R_xlen_t size = 100000;

  cpp11::writable::strings x(size);

  cpp11::r_string elt(NA_STRING);

  for (R_xlen_t i = 0; i < size; ++i) {
    x[i] = elt;
  }

  return x;
}

bench::mark(string_proxy_assignment_(), iterations = 100)
#> # A tibble: 1 × 6
#>   expression                      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>                 <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 string_proxy_assignment_()   14.9ms     15ms      66.6     781KB    0.673

bench::mark(string_proxy_assignment_(), iterations = 100)
#> # A tibble: 1 × 6
#>   expression                      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>                 <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 string_proxy_assignment_()    344µs    360µs     2769.     781KB     28.0

---

[[cpp11::register]] cpp11::writable::strings string_push_back_() {
  R_xlen_t size = 100000;

  cpp11::writable::strings x;

  cpp11::r_string elt(NA_STRING);

  for (R_xlen_t i = 0; i < size; ++i) {
    x.push_back(elt);
  }

  return x;
}

bench::mark(string_push_back_(), iterations = 100)
#> # A tibble: 1 × 6
#>   expression               min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>          <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 string_push_back_()   20.5ms   21.8ms      46.1    2.76MB     2.43

bench::mark(string_push_back_(), iterations = 100)
#> # A tibble: 1 × 6
#>   expression               min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>          <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 string_push_back_()   5.17ms   5.33ms      185.    2.76MB     9.73