move implementation into beman::inplace_vector namespace

[20162026]

close #85

namespace changes:
beman::inplace_vector<..> moved to beman::inplace_vector::inplace_vector<..>
beman::details::inplace_vector moved to beman::inplace_vector::details
beman::freestanding::inplace_vector to beman::inplace_vector::freestanding
beman::from_range polyfill moved to beman::inplace_vector::details::from_range
beman::erase moved to beman::inplace_vector::erase

this will couse some minor inconvenience for anyone who is already using the library, but I do believe we should fix the namespace for consistency

also add missing test case for resize with init value #89

[wusatosi]

Thank you!

[20162026]

@wusatosi if there are no objections, I will merge this tomorrow

[JeffGarland]

LGTM. One thing to consider is that godbolt examples might start to fail after this change, but that can be cleaned up after this commit is synced there.