Update TOML Fortran to latest version

[awvwgk]

• replace deprecated toml_parse with toml_load
• update from using toml_serializer visitor to toml_serialize function
• use more readable syntax in manifest (previous syntax was used due to tableMap Codec with implicit tables not parsed kowainik/tomland#336 in Haskell version)

[urbanjost]

The only comment is writing messages to stdout, which I think should all go to stderr, but that is mixed throughout fpm and does not have anything to do with this change specifically. Tried it with 635 fpm.toml files (albeit a lot of them are very similiar) and saw nothing abnormal. Looks good to me.