Use syntax-propertize-function set in submodes

WIP, it visibly slows down parsing.
Define mmm-syntax-propertize-function

This is faster, but still kinda slow.
* mmm-syntax-propertize-function: Use font-lock-syntactic-keywords
  with less fortunate submodes.
* Don't clear text properties, syntax-propertize already does that for us.
* mmm-apply-all: flush syntax cache
Revert c548593, for easier optimization

* mmm-fontify-region-list: Don't set global and buffer vars when the submode
  hasn't changed. Gives us nice perfomance boost.
Rewrite mmm-regions-alist for performance

Not much improvement here, but at least we don't iterate multiple times over the
same list now.
* mmm-regions-in: Keep overlay references in the return value
* mmm-fontify-region-list, mmm-syntax-propertize-function: Use them.
* mmm-erb-scan-region: Same.
* mmm-submode-overlay-at: Remove, not used anymore.

And bump the version, this is a change in the API.
Hopefuly, a relatively safe one.
Rebinding syntax-propertize-chunk-size is pointless

It's only used in the caller, to determine STOP.
Merge pull request #21 from dgutov/syntax-propertize

Define a composite syntax-propertize-function