extend static_variant with is_type() method

[crypto-ape]

Hey Monkeys!

This PR implements the proposal from the issue #1769 .

Ape out!

[oxarbitrage]

This pull is not building, please check travis output: https://travis-ci.org/bitshares/bitshares-core/builds/544288497?utm_source=github_status&utm_medium=notification

[crypto-ape]

The actual feature is implemented within the fc library, I was just about tu make a PR there.
Please, how do you await such a necessary change from a submodule to be provided other than in custom PR within that submodule?

[oxarbitrage]

Need to send pull request to FC, after approved and merged a bump FC commit should be included in this one.

[crypto-ape]

Okay. Here it goes.
bitshares/bitshares-fc#138

[oxarbitrage]

merged bitshares/bitshares-fc#138 , please bump FC here.

for example, last merged bump is included in #1768 at commit: 90efe68

[abitmore]

@crypto-ape commands:

cd libraries/fc
git checkout master
git pull
cd ..
git add fc
git commit

[oxarbitrage]

thank you :)

[crypto-ape]

Done.