feat(version): Determine binary version during initalization

This ensures that checks for features should work, and bails early if
that is not the case, since if this fails the binary is essentially
broken. This also simplifies code that performs these checks, ie instead
of:

    binaryVersion, err := version.GetReleaseVersion()
    if err != nil {
      // handle error? panic?
    }
    if version.SupportsFeature(binaryVersion, version.FeatureA) {
      // code for FeatureA
    } else {
      // without FeatureA
    }

Implementations can do:

    if version.SupportsFeature(version.Binary, version.FeatureA) {
      // code for FeatureA
    } else {
      // without FeatureA
    }

version/feature_manager.go

@@ -27,16 +27,29 @@ const (
 
 var featureMap map[Feature]MetadataConstraint
 
+// Binary is the version of the running binary.
+// This can be used for feature checks.
+var Binary *gvers.Version
+
 func init() {
+	// Do this early to ensure version is valid, if this panics something is
+	// very broken with the version and any version checks based on the binary's
+	// version info will not work correctly. Also do this once since the version
+	// can't change while running.
+	var err error
+	Binary, err = GetReleaseVersion()
+	if err != nil {
+		panic(err)
+	}
+
 	if featureMap == nil {
 		featureMap = make(map[Feature]MetadataConstraint)
 	}
 	/*
 		Add constraints here following this format after adding a Feature to the Feature iota:
-		featureConstraint, err := gvers.NewConstraint(">= 0.1.0") // This feature exists at 0.1.0 and above
 		featureMap[FEATURE] = MetadataConstraint{
 			MetaInfo:    []Metadata{OSS, HCP},
-			Constraints: featureConstraint,
+			Constraints: mustNewConstraints(">= 0.1.0"), // This feature exists at 0.1.0 and above
 		}
 	*/
 }
@@ -49,6 +62,14 @@ func metadataStringToMetadata(m string) Metadata {
 	return OSS
 }
 
+func mustNewConstraints(v string) gvers.Constraints {
+	c, err := gvers.NewConstraint(v)
+	if err != nil {
+		panic(err)
+	}
+	return c
+}
+
 // Check returns a bool indicating if a version meets the metadata constraint
 // for a feature. Check returns false if version is nil.
 func (m MetadataConstraint) Check(version *gvers.Version) bool {