Assorted Improvements

[jdgleaver]

This PR makes the following changes to the core:

• All direct file access has been replaced with VFS routines
• At present, the core sets need_fullpath = true. When using frontends that support the RETRO_ENVIRONMENT_SET_CONTENT_INFO_OVERRIDE callback, this restriction is now removed, allowing content to be loaded via a frontend data buffer
• At present, the core will not load content unless a BIOS file is present, but the BIOS file is not actually used at all (moreover, the core will crash if you do enable the BIOS loading code). BIOS loading is now fixed, and the BIOS file has been made optional
• EEPROM save files are now generated in the frontend save directory, instead of the content directory
• At present the usage of the audio sample batch callback is bizarre and inefficient. This has been fixed.
• All logging is now performed via the frontend logging callback

[lgtm-com[bot]]

This pull request introduces 2 alerts when merging 73b26adfad4e862a3c13c7315364a0299afad47c into 39a84f93e839f22a8c1f5ea35b80e910183476ce - view on LGTM.com

new alerts:

• 2 for FIXME comment

[inactive123]

LGTM.