circleCI config and linting fixes (#3)

* Fixed circleci config and fixed linting warnings

* Updated circleCI for go version 1.12 and split jobs for build/testing

* updated go version to 1.12.5 for circleCI

* Go mod tidy dependencies

* Updated linting tools and cleared up code lint smells

* Added workflow to run build and test jobs

* Moved linting command to build workflow

* Get dependencies before linting by default

* Added go module flag to linter and increased deadline to pull packages

core/chain.go

@@ -26,6 +26,8 @@ type ChainContext struct {
 	headersByNumber map[uint64]*ethtypes.Header
 }
 
+// NewChainContext generates new ChainContext based on Ethereum's core.ChainContext and
+// consensus.Engine interfaces in order to process Ethereum transactions.
 func NewChainContext() *ChainContext {
 	return &ChainContext{
 		headersByNumber: make(map[uint64]*ethtypes.Header),