#5834 Fix most encoding problems with Guile 2.x

[Jonas Hahnfeld]

Fix most encoding problems with Guile 2.x

Individual commits:
1) Treat possibly incomplete UTF-8 as binary

replace_special_characters checks that the substring doesn't start
mid-UTF-8, but it does not guarantee that it ends in a complete glyph.
So just explicitly treat it as binary when creating the SCM.
While modifying the function, avoid comparison of zero-length substrings.

2) Use UTF-8 for all conversions to / from Scheme

LilyPond really expects all input to be encoded in UTF-8, and we should
not let GUILE 2.x mangle with it.

http://codereview.appspot.com/555420043

[Anonymous]

Passes make, make check and a full make doc.

(note I am testing with only guile 1.8 installed - if that matters?)

[Jonas Hahnfeld]

No, this is fine: Guile 1.8 stays the default for now and the compatibility defines make sure that the change only affects Guile 2.x

[Anonymous]

Patch on coumntdown for March 11th.

[Anonymous]

Patch counted down, please push.

[Jonas Hahnfeld]

commit f756cab383d91faa6430a1c7f1b041f7855721a9
Author:     Jonas Hahnfeld <hahnjo@hahnjo.de>
AuthorDate: Sat Mar 7 19:44:48 2020 +0100
Commit:     Jonas Hahnfeld <hahnjo@hahnjo.de>
CommitDate: Wed Mar 11 08:24:23 2020 +0100

    Issue 5834/2: Use UTF-8 for all conversions to / from Scheme

    LilyPond really expects all input to be encoded in UTF-8, and we should
    not let GUILE 2.x mangle with it.

commit cbac8d5d3113b2b224956b555c147ccc230ab14a
Author:     Jonas Hahnfeld <hahnjo@hahnjo.de>
AuthorDate: Sat Mar 7 00:03:15 2020 +0100
Commit:     Jonas Hahnfeld <hahnjo@hahnjo.de>
CommitDate: Wed Mar 11 08:24:23 2020 +0100

    Issue 5834/1: Treat possibly incomplete UTF-8 as binary

    replace_special_characters checks that the substring doesn't start
    mid-UTF-8, but it does not guarantee that it ends in a complete glyph.
    So just explicitly treat it as binary when creating the SCM.
    While modifying the function, avoid comparison of zero-length substrings.