Fix incorrect regexp matches

There are some cases where the '.' character would also match any
character and could lead to incorrect results. For example the
regular expression -  `^ioutils.WriteFile$' would match
ioutils.WriteFile, but also ioutils_WriteFile.

Additionally made sure that all regexp were declared using raw
strings to avoid any unnecesary string escaping that potentially
make the regexp difficult to read.

rules/rand.go

@@ -41,7 +41,7 @@ func (w *WeakRand) Match(n ast.Node, c *gas.Context) (*gas.Issue, error) {
 
 func NewWeakRandCheck() (r gas.Rule, n ast.Node) {
 	r = &WeakRand{
-		pattern:     regexp.MustCompile(`^rand.Read$`),
+		pattern:     regexp.MustCompile(`^rand\.Read$`),
 		packageName: "rand",
 		packagePath: "math/rand",
 		MetaData: gas.MetaData{