Rename: field type -> field class

This patch renames everything named "field type" to "field class",
including function names, variable names, file names, metadata AST
names, code comments, log messages, and more.

Since everything else in the API is named "class" rather than "type"
(event class, stream class, component class, clock class, etc.), this
makes the API more consistent and easier to document. This is also in
line with CTF 2's terminology.

API documentation, man pages, Python bindings, and some plugins are left
untouched because they will change in the future anyway.

The CTF writer still uses the "field type" terminology.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>