Remove C code

[dieghernan]

Hi:

Following #46 and related to #45 , this PR removes the C code of the package, substituting it by pure R (base) code. I am aware this is a critical change to the package, so let me share some points of attention on this PR:

• Version bump to make clear that this is a dev version
• trimws has been updated to the latest version of the same function on R, see https://github.com/wch/r-source/blob/5ab79ec84040684c74dc9c901fde944fff6e8375/src/library/base/R/strwrap.R#L219-L229
• Some parameters have been deprecated:
  • src in do_read_bib() is not needed any more
  • read.bib(): header and footer. They were never actually used, see: https://github.com/ropensci/bibtex/blob/05a401cda1d4f1757a82979d181c4486a54755b1/R/bibtex.R#L162-L170
• A whole set of functions have been added to utils.R. This is actually the replacement of C in R code.
• An url on README has been updated following the urlcheck package.
• There is a change on a snapshot. Actually now is correct, before was ommiting a part on the string:

  
  @String{Rnews = "http://CRAN.R-project.org/doc/Rnews/" }
  @String{Rnews2001-1 = rnews # "Rnews_2001-1.pdf"}
  @String{Rnews2001-2 = rnews # "Rnews_2001-2.pdf"}
  @String{Rnews2001-3 = rnews # "Rnews_2001-3.pdf"}

So `Rnews2001-3` should be parsed as  `rnews # "Rnews_2001-3.pdf` (http://CRAN.R-project.org/doc/Rnews/Rnews_2001-3.pdf)

- I run `revdepcheck::revdep_check()` succesfully. This is specially important for maintainers of the affected packages (@renozao, @sckott, @njahn82, @bwiernik, @mwmclean )

Comments welcome

This would also close #42 , fix #16

[dieghernan]

I added some comments on the review tab to make easier to understand some things @coatless

[dieghernan]

Thanks for this useful review. I would set this as a Draft until I tackle all your comments

[dieghernan]

Hi @coatless , I think I tackled all your comments, ready for review

[dieghernan]

Hi @coatless , just a quick reminder on this

[maelle]

:wave: @coatless @dieghernan, what's the status on this? Thanks both for your work on {bibtex} :pray:

[dieghernan]

Hi @maelle

I re-runned checks, examples and snapshots after 8 months and I see no problems, so this is ready on my side. This PR would close #16 and close #42

[maelle]

Thanks @dieghernan! @coatless would you be able to review this PR?

[coatless]

@dieghernan thanks for addressing the remaining issues. Sorry this sat for so long!