Minor code cleanup questions

[MeanSquaredError]

While reading the new PR that adds NULLS FIRST/NULLS LAST, I came across a couple of places in the code that looked a slightly suspicious, so I decided to open a new issue about them:

1. Comments about CRTP. There are several structures that have comments in them that they should be used in CRTP:

• sqlpp23/include/sqlpp23/core/aggregate_function/enable_over.h

  Line 33 in f3f344d

  // To be used as CRTP base for aggregate expressions that should offer the

• sqlpp23/include/sqlpp23/core/operator/enable_comparison.h

  Line 33 in f3f344d

  // To be used as CRTP base for expressions that should offer the comparison

• sqlpp23/include/sqlpp23/core/operator/enable_as.h

  Line 35 in f3f344d

  // To be used as CRTP base for expressions that should offer the as() member

  
  All these comments about CRTP seem to be obsolete and probably should be updated. None of these structures are actually used with the CRTP pattern. I guess CRTP has been replaced by deducing this?

2. There are a multiple places where various structures inherit from enable_comparison which means that they all get the asc() and desc() methods. E.g. type cast expressions, case expressions and string concat expressions inherit from enable_comparison. I haven't actually tried with an actual test program, but given that they all inherit from enable_comparison, it seems to me that they all get the asc() and desc() methods. Aren't there too many expressions that get these methods? E.g. all of the ones that I listed above shouldn't get asc() and desc().