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/unsafe.go

@@ -34,7 +34,7 @@ func (r *UsingUnsafe) Match(n ast.Node, c *gas.Context) (gi *gas.Issue, err erro
 
 func NewUsingUnsafe() (r gas.Rule, n ast.Node) {
 	r = &UsingUnsafe{
-		pattern: regexp.MustCompile("unsafe.*"),
+		pattern: regexp.MustCompile(`unsafe.*`),
 		MetaData: gas.MetaData{
 			What:       "Use of unsafe calls should be audited",
 			Severity:   gas.Low,