Issue 991 final changes (#1350)

* Issue 991 final changes

1) price default constructor is allowed again, because of fc::variant::as method needs. 2) price(const asset&, const asset&) constructor is declared as explicit, and passed arguments must be filled. 
3) Added asset operator * ( const asset& a, const asset& b ), needed after blocking implicit convertion to price (previously asset operator *  ( const asset& a, const price& b ); was chosen).

* 991-explicit price-constructor fixes to pass chain_tests

- eliminated one of previously added assertions, be too strict, even passed data can lead to creation of invalid price object.
- updated comments

* Updated submodule libraries/fc

* Required changes related to price validation 

price explicit constructor can throw an exception now when passed args are broken
Extended operation_tests by case when validation is done directly inside explicit price constructor

* Revert "Updated submodule libraries/fc"

This reverts commit 6b0f8f80bbcfc725e4dd06dcf64bae18654da8fe.