Check constancy when calling to external function

[charles-cooper]

What I did

Fix #1468 , #1377

How I did it

The compiler was not checking constancy of external calls inside of a constant context. However, it was correctly generating static calls, so calls to modifying functions would fail at runtime when the callee tried to modify state (per https://github.com/ethereum/EIPs/blob/d95612c8206b7e225bec5dc34c4b019c9531e44d/EIPS/eip-214.md). This adds a check when compiling external calls that they cannot be modifying within a constant context.

How to verify it

Description for the changelog

Add constancy check for external calls

Cute Animal Picture

[jacqueswww]

Add a test in /syntax/ and it's good to go.

[charles-cooper]

Note to self: we decided at last meeting to add an assert_no_modify (or something) to the language and so this PR needs to be expanded to add that. Also I need to fix the tests

[charles-cooper]

@jacqueswww should be ready to go

[charles-cooper]

pinging @fubuloubu @davesque for review

[fubuloubu]

Need to add some docs on assert_modifiable but I don't see why this cannot be merged now.

[jacqueswww]

LGTM