feat: add fputc wrapper to IO

[scolsen]

fputc is a low-level function that writes a single C character to a file. We can use this as a basis for more elegant APIs.

The signature matches that of the C standard library function for compatibility, which takes an Int instead of a char. However, the documentation notes that the int argument is converted to an unsigned char.

[scolsen]

Looks like the Linux and MacOS CI errors were transient errors related to downloading GHC