std: Second pass stabilization for string

[alexcrichton]

This commit performs a second pass over the std::string module, performing the
following actions:

• The name std::string is now stable.
• The String::from_utf8 function is now stable after having been altered to
  return a new FromUtf8Error structure. The FromUtf8Error structure is now
  stable as well as its into_bytes and utf8_error methods.
• The String::from_utf8_lossy function is now stable.
• The String::from_chars method is now deprecated in favor of .collect()
• The String::from_raw_parts method is now stable
• The String::from_str function remains experimental
• The String::from_raw_buf function remains experimental
• The String::from_raw_buf_len function remains experimental
• The String::from_utf8_unchecked function is now stable
• The String::from_char function is now deprecated in favor of
  repeat(c).take(n).collect()
• The String::grow function is now deprecated in favor of
  .extend(repeat(c).take(n)
• The String::capacity method is now stable
• The String::reserve method is now stable
• The String::reserve_exact method is now stable
• The String::shrink_to_fit method is now stable
• The String::pop method is now stable
• The String::as_mut_vec method is now stable
• The String::is_empty method is now stable
• The IntoString trait is now deprecated (there are no implementors)
• The String::truncate method is now stable
• The String::insert method is now stable
• The String::remove method is now stable
• The String::push method is now stable
• The String::push_str method is now stable
• The String::from_utf16 function is now stable after its error type has now
  become an opaque structure to carry more semantic information in the future.

A number of these changes are breaking changes, but the migrations should be
fairly straightforward on a case-by-case basis (outlined above where possible).

[breaking-change]

[rust-highfive]

r? @brson

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[alexcrichton]

r? @aturon