Deal with namespace collisions for columns and Dataframe functions

[charlesgregory]

For now we prevent compilation with specified function names as column names.

koalas/source/koalas/dataframe.d

Lines 25 to 36 in 19f891c

	static foreach(i,member;memberNames){
	static assert(member.stringof != "length");
	static assert(member.stringof != "shape");
	static assert(member.stringof != "copy");
	static assert(member.stringof != "columns");
	static assert(member.stringof != "sort");
	static assert(member.stringof != "head");
	static assert(member.stringof != "toString");
	static assert(member.stringof != "unique");
	static assert(member.stringof != "records");
	mixin("alias "~member.stringof~" = getCol!\""~member.stringof~"\";");
	}

This code allows to retrieve a column like this df.col. We want to avoid situations where df.length as a column collides with length property function.

koalas/source/koalas/dataframe.d

Lines 43 to 45 in 19f891c

	@property ulong length(){
	return records.length;
	}

[jblachly]

should you in some, or all, cases add a prefix char like _ to avoid collision. for example, df._length ?

[charlesgregory]

I think that's reasonable solution as long as its consistent for all columns.