make filehandle() check lexical open hint from caller
Implement locking as part of filehandle method

All locking operations are now consolidated in the filehandle method,
simplifying input-output methods a bit.

Opening a filehandle to write in locked mode needs to defer any
truncation until after the lock is acquired, but open() can't do that.
We use sysopen and some PerlIO hackery to emulate a non-truncating
open() in such case.

Unnecessary seek and unlock calls have been removed.
document that filehandle respects caller's lexical open pragma
document Types::Path::Tiny
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