Improve warning message for missing includes

[Pennycook]

Related issues

Closes #50.

I adopted a slightly different solution than that outlined in #50, after offline discussion with @laserkelvin. Rather than print a large number of suggested actions as part of each individual warning message, we can use a meta-warning to describe suggested fixes just once.

Proposed changes

• Clarify that the warning pertains to a missing include.
• Differentiate between user and system includes.
• Show the line of code that triggered the warning.

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A quick example of what this looks like:

[WARNING ] /path/to/source/file.cpp:77: user include 'float.h' not found
   77 | #include "float.h"
[WARNING ] /path/to/source/file.cpp:79: system include 'cstring' not found
   79 | #include <cstring>

Writing "user" or "system" in the message is mainly there to help us build a WarningAggregator that can give different advice in both cases, but it should also be helpful for anybody greping through the logs.