Do not duplicate prototypes between rinterface.c and rinterface_extra.c

[szhorvat]

rinterface.c and rinterface_extra.c have a lot of function prototypes literally inserted.

TODO:

• Move these to a common header, and include that header in both files
• Remove unused functions
• Remove unused prototypes
• Change int return types to igraph_error_t where necessary. See fix: check that we receive a scalar when expecting a scalar in C code #1051 (comment)

The current setup is a crash risk and increases maintenance costs.

@krlmlr @Antonov548 can either of you do this please?

[krlmlr]

Done.

[github-actions]

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