Clean branch to compile with LFortran

[certik]

The list of all workarounds in this PR:

• [x] https://github.com/lfortran/lfortran/issues/2323
• [x] https://github.com/lfortran/lfortran/issues/2311
• [ ] The ./test_chat test segfaults
• [ ] The chat executable doesn't run.
• [ ] https://github.com/lfortran/lfortran/issues/1999
• [x] The macOS LFortran 0.20.1 conda works, but Linux can't find the package: resolve the problem (just took longer to be available in the channel --- everything works now)
• [x] Make a new conda release. The latest master diff is just this: https://github.com/certik/fastGPT/compare/lf29run?expand=1 (the three open issues below); new conda package here: https://github.com/conda-forge/lfortran-feedstock/pull/43; update this CI to use it
• [x] Compile all files in Release mode, run two tests; fixed by: https://github.com/lfortran/lfortran/pull/2335
• [x] character -> integer(1): https://github.com/lfortran/lfortran/issues/2324
• [x] https://github.com/lfortran/lfortran/issues/2321; fixed by https://github.com/lfortran/lfortran/pull/2329
• [x] https://github.com/lfortran/lfortran/issues/2320; fixed by https://github.com/lfortran/lfortran/pull/2329
• [x] Format: f4.2 -> f8.2: https://github.com/lfortran/lfortran/issues/2325
• [x] tokens = [intokens(:idx-1), merged_token, intokens(idx+2:)]
• [x] More robust merge_utf8_pairs
• [x] More robust encode
• [x] Several decode workarounds

[Thirumalai-Shaktivel]

The ./test_chat test segfaults, because we don't allocate inputs for array constant. Should we add an allocate statement to the upstream? or Fix it in the LFortran?

[certik]

Push a new branch that works. Then we'll figure out what to do about it.

[certik]

Closing in favor of #72.