~mna/snow unlisted

snow/cmd d---------
Merge branch 'wip-interfaces'
pkg/codegen: test interface code generation
Merge branch 'wip-generics'
cleanup some todo comments
pkg/codegen: fix initialization of fields in struct init
pkg/codegen: struct init calls work, although not in the expected way yet
Merge branch 'wip-post-ast'
pkg/{codegen,semantic}: OMG all codegen tests pass, all is good
pkg/typecheck: remove typecheck package
pkg/semantic: finish type-assign pass, update tests
Merge branch 'wip-tuple'
pkg/codegen: generate code for tuple type and expr
pkg/codegen: generate struct methods and self accesses
Merge branch 'wip-codegen-goast'
pkg/codegen: finish naive/incorrect translation pass
pkg/codegen: translate more nodes to Go AST
pkg/codegen: start refactor in 4 steps

mangle is probably done;
translate needs to happen;
add positions needs to be completed
pkg/parser: parse comparison and logical ops
remove dependency on stretchr/testify
cmd/snowc: add typecheck command implementation
cmd/snowc: support position mode and extra args for build flags