Warn about unused non-record function arguments

[poscat0x04]

Proposed feature

Add a warning for simple (non-record, i.e., not of the form { xxx }) function arguments that

1. Does not have underscore (_) preceding its name
2. Not used in the function body

For example, the following code will have this warning (on the variable y).

x: y: x

While this code

x: _: x

or this code

x: _y: x

will not.

[oxalica]

Add a warning for simple (non-record, i.e., not of the form { xxx }) function arguments that

1. Does not have underscore (`_`) preceding its name

2. Not used in the function body

_ starting identifiers are now ignored from liveness check. Note that { a }: 42 will still not trigger a warning since it's different from _: 42: the former requires the argument to be in the given form, or it will throw even without evaluating it.