#291 Add optional specifier 'Explicit' for constructor declaration to inhibit use by compiler in implicit conversions

[fxm (freebasic.net)]

Optionnal specifier in order to forbid the constructor use by compiler in implicit conversions (only an explicit use of constructor is allowed).

Proposed syntax:
Declare Constructor (...) Explicit

For more explanation on its utility, refer to forum at the synthesis post:
http://www.freebasic.net/forum/viewtopic.php?p=203141#p203141
(see links to the two closed bug reports:

• http://sourceforge.net/p/fbc/bugs/646/ : #646 Illicit call of copy constructor when passing argument byref
• http://sourceforge.net/p/fbc/bugs/653/ : #653 Illicit construction (byref) or redundant construction (byval)
  and to other discussion posts)

[fxm (freebasic.net)]

This feature is available in C ++, by also using the qualifier 'explicit' but ahead constructor declaration.

[fxm (freebasic.net)]

As in C++, this qualifier "Explicit" should also be usable with the operator "Cast()".

See forum at post:
http://www.freebasic.net/forum/viewtopic.php?p=203357#p203357