refactor: make dc_dehtml() function safe

* Make dc_dehtml() function safe

* Change type of is_msgrmsg parameter to bool

* Narrow type of local variable in simplify_plain_text()

* Export less fields of `Simplify' record

* Demote is_cut_* from fields of `Simplify' to local variables

* Refactor part of simplify_plain_text()

Refactor footer ("-- " and similar) code into separate function,
and re-implement it with standard Rust string methods.

It simplifies code and allows removing one mutable local variable.

* Replace dc_split_into_lines with String.split()

  * src/dc_simplify.rs(find_message_footer): adjust type signature to accept
    slice of &str, not slice of pointers

  * src/dc_simplify.rs(simplify_plain_text): adjust code to use '==' operator
    instead of strcmp(3).

  * src/dc_simplify.rs(is_empty_line, is_quoted_headline, is_plain_quote):
    + adjust type signatures to accept &str, not 'const char *'
    + remove no longer needed 'unsafe' qualifier

  * src/dc_tools(dc_split_into_lines, dc_free_splitted_lines): remove no longer
    used functions.

In addition to additional type-safety, this change reduces number of
allocations: String.split returns iterator of &str.

* Make simplify_plain_text() safe

* Make Simplify.simplify return String, not pointer

* Refactor Simplify.simplify to use String methods, not pointers

* Make Simplify.simplify() safe

* Avoid neeless allocation in Simplify.simplify when input is html

* Add tests for simplify utilities

* Document discussion about is_empty_line() discussion

src/dc_dehtml.rs

@@ -2,9 +2,6 @@ use lazy_static::lazy_static;
 use quick_xml;
 use quick_xml::events::{BytesEnd, BytesStart, BytesText};
 
-use crate::dc_tools::*;
-use crate::x::*;
-
 lazy_static! {
     static ref LINE_RE: regex::Regex = regex::Regex::new(r"(\r?\n)+").unwrap();
 }
@@ -24,19 +21,20 @@ enum AddText {
 
 // dc_dehtml() returns way too many lineends; however, an optimisation on this issue is not needed as
 // the lineends are typically remove in further processing by the caller
-pub unsafe fn dc_dehtml(buf_terminated: *mut libc::c_char) -> *mut libc::c_char {
-    dc_trim(buf_terminated);
-    if *buf_terminated.offset(0isize) as libc::c_int == 0i32 {
-        return dc_strdup(b"\x00" as *const u8 as *const libc::c_char);
+pub fn dc_dehtml(buf_terminated: &str) -> String {
+    let buf_terminated = buf_terminated.trim();
+
+    if buf_terminated.is_empty() {
+        return "".into();
     }
 
     let mut dehtml = Dehtml {
-        strbuilder: String::with_capacity(strlen(buf_terminated)),
+        strbuilder: String::with_capacity(buf_terminated.len()),
         add_text: AddText::YesRemoveLineEnds,
         last_href: None,
     };
 
-    let mut reader = quick_xml::Reader::from_str(as_str(buf_terminated));
+    let mut reader = quick_xml::Reader::from_str(buf_terminated);
 
     let mut buf = Vec::new();
 
@@ -61,7 +59,7 @@ pub unsafe fn dc_dehtml(buf_terminated: *mut libc::c_char) -> *mut libc::c_char
         buf.clear();
     }
 
-    dehtml.strbuilder.strdup()
+    dehtml.strbuilder
 }
 
 fn dehtml_text_cb(event: &BytesText, dehtml: &mut Dehtml) {