Re-enable all warning/errors and fix the underlying code issues #37

no-psabi, no-sign-compare, no-unknown-pragma and -flax-vector-conversions should be removed since xvec has no control over the flags that the end user of the library might want. the code should compile without any disabled warnings.

psabi, lax-vector-conversions: Needs stricter control over ABIs - specifying the vector type explicitly in more places rather than relying on implicit conversions.
no-sign-compare: generator-ctor is passing unsigned index, not simd_size_type
no-unknown-pragma: Forced unrolling inside operators doesn't work in gcc as expected