linear/tvec1: added `(void)i;` in array subscript operator to appease -Wunused-parameter.
Appeased the -pedantic gods¹; tidy.

1: In config, detail/hash/fnv_impl, detail/hash/murmur_impl, and
detail/linear/type_traits.
test: replaced assert() with forced assertion (fassert()); tidy.
test/hash/validate: NOP constexpr murmur3 tests on GCC.¹

1: For some reason they don't like our pointer arithmetic.
README: added Building and Defects sections; wrapped to 80 columns.