loadtxt/savetxt: do not require space after last entry

[perazz]

Attempt to address #862.

The current formats require an end-of-row space both in savetxt and loadtxt, i.e.,

format(*(i0,1x))

This PR proposes to not require a space after the last record, i.e.,

format(*(i0,:,1x))

This should hopefully improve the seldom crashing CI tests.
@fortran-lang/stdlib @chuckyvt @minhqdao

PS: these routines would benefit from some form of error control in the read and write statements.
Without changing the API, I propose checking for iostat and iomsg, so that when something goes wrong, an error can be produced to give information about what happened (what file, which line).