Add config options for string hash changes

- Add DUK_USE_STRHASH_SKIP_SHIFT option for the "fast" skipping hash.

- Add DUK_USE_STRHASH_DENSE option (default false) to revert back to
  the Duktape 1.3.0 string hash algorithm.

config/config-options/DUK_USE_STRHASH_DENSE.yaml

@@ -0,0 +1,10 @@
+define: DUK_USE_STRHASH_DENSE
+introduced: 1.4.0
+default: false
+tags:
+  - performance
+  - sandbox
+description: >
+  Use the slower but more dense string hash algorithm from Duktape 1.3.0 and
+  prior (based on Murmurhash2).  This may be useful if you're experiencing
+  collision issues with the default hash algorithm.

config/config-options/DUK_USE_STRHASH_SKIP_SHIFT.yaml

@@ -0,0 +1,12 @@
+define: DUK_USE_STRHASH_SKIP_SHIFT
+introduced: 1.4.0
+default: 5
+tags:
+  - performance
+description: >
+  Shift value to use for string hash skip offset when using the default
+  (fast) string hash.  The skip offset is calculated as:
+  ((length >> DUK_USE_STRHASH_SKIP_SHIFT) + 1).  A higher value will be
+  slower but sample the string more densely.
+
+  You should only change this if you run into issues with the default value.

config/examples/performance_sensitive.yaml

@@ -15,4 +15,6 @@ DUK_USE_JSON_DECNUMBER_FASTPATH: true
 DUK_USE_JSON_EATWHITE_FASTPATH: true
 DUK_USE_INTERRUPT_COUNTER: false
 DUK_USE_DEBUGGER_SUPPORT: false
+DUK_USE_STRHASH_DENSE: false
+DUK_USE_STRHASH_SKIP_SHIFT: 5  # may be able to reduce
 #DUK_USE_EXEC_FUN_LOCAL: test both values, marginal benefit

config/examples/security_sensitive.yaml

@@ -10,4 +10,8 @@ DUK_USE_VERBOSE_PROP_ERRORS: false
 # code).
 DUK_USE_TRACEBACKS: false
 
+# Dense string hashing may be useful against accidental string hash collisions.
+# This won't prevent an attacker from finding intentional collisions.
+DUK_USE_STRHASH_DENSE: true
+
 # TBD