Implement binary output and merge mode

We implement a merge mode, which takes the results of multiple runs
and merges them. On an analysis-by-analysis basis we decide whether a
diagnostic has to occur in all runs to be merged. In addition, we take
into consideration whether a file was part of a run in the first
place, to avoid incorrectly dropping diagnostics.

We add a new output format called "binary", which writes diagnostics
and metadata using gob. This data is the input to the merge mode.

Currently, the metadata includes a list of all files that were
checked. In the future, we might change to amore finegrained data,
such as which lines of code were reachable in the IR.

Updates gh-938

analysis/lint/lint.go

@@ -55,6 +55,13 @@ const (
 	SeverityHint
 )
 
+type MergeStrategy int
+
+const (
+	MergeIfAny MergeStrategy = iota
+	MergeIfAll
+)
+
 type RawDocumentation struct {
 	Title      string
 	Text       string
@@ -64,6 +71,7 @@ type RawDocumentation struct {
 	NonDefault bool
 	Options    []string
 	Severity   Severity
+	MergeIf    MergeStrategy
 }
 
 type Documentation struct {
@@ -79,6 +87,7 @@ type Documentation struct {
 	NonDefault bool
 	Options    []string
 	Severity   Severity
+	MergeIf    MergeStrategy
 }
 
 func Markdownify(m map[string]*RawDocumentation) map[string]*Documentation {
@@ -97,6 +106,7 @@ func Markdownify(m map[string]*RawDocumentation) map[string]*Documentation {
 			NonDefault: v.NonDefault,
 			Options:    v.Options,
 			Severity:   v.Severity,
+			MergeIf:    v.MergeIf,
 		}
 	}
 	return out