C: Introduce string_equals utility function

[citizen428]

Note: this was built on top of #1129, which should be merged first. I didn't want to update the already removed visit subcommand.

This PR introduces a string utility function called string_equals and updates usages of strcmp to use that instead.

Rationale:
While perusing main.c, I found the repetitive strcmp calls a bit of a nuisance. I originally considered adding the following macro:

#define STRING_EQ(a, b) (strcmp((a), (b)) == 0)

However, since we're targeting C99, we can also use a static inline function instead.

File size considerations:
I built the binary on my machine with both the strcmp and string_equals versions. As expected, with our production flags, this resulted in no increased binary size as clang successfully inlines all the calls. For debug builds, there's an increase of 448 bytes, which I think is a reasonable tradeoff for the increased readability.

Build	strcmp	string_eq	Diff
Debug	841,208 bytes	841,656 bytes	+448 bytes (+0.05%)
Production	649,960 bytes	649,960 bytes	0 bytes

Open question:
Are we actually using the production flags to build releases? The assigned variable is never used in the Makefile.

[citizen428]

Note that this is a header-only definition, which is common for this type of static inline function to ensure every compilation unit has its copy of the function.

[citizen428]

@marcoroth Renamed everything as requested.

[marcoroth]

Thank you @citizen428! 🙏🏼