Added sext:partial_decode/1 + corresponding EQC properties

Adapted sext_eqc so that it will use Proper if EQC not available.
Some tests fail with Proper for some reason. Needs investigating.

sext_eqc now checks the OS env var SEXT_TESTS. For example:

SEXT_TESTS=100 make test

will make QuickCheck run batches of 100 tests for each property.