Implement parser that supports unsupported features

[ordinarymath]

When writing CakeML code/porting code from other ml implementations one would accidentally use features that CakeML currently doesn't support. This issue is not proposing to implement these features but make the parser successfully parse said features and output error messages along the lines that the feature isn't supported.

[wildptr]

Three major features I can think of:

1. Datatype definition syntax (C of t1 * t2 * ...)
2. Records
3. Functors and signatures

Anything else?

[ordinarymath]

open infix