* Add streaming support to save for ExternalStream data
Prior to this change, save would collect data from an ExternalStream (data
originating from externals) consuming memory for the full amount of data piped
to it,
This change adds streaming support for ExternalStream allowing saving of
arbitrarily large files and bounding memory usage.
* Remove broken save test
This test passes but not for the right reasons, since this test was
written filename has become a required parameter. The parser outputs
an error but the test still passes as is checking the original un-mutated
file assuming save has re-written the contents.
This change removes the test.
```
running 1 test
=== stderr
Error: nu::parser::missing_positional (https://docs.rs/nu-parser/0.60.0/nu-parser/enum.ParseError.html#variant.MissingPositional)
× Missing required positional argument.
╭─[source:1:1]
1 │ open save_test_1/cargo_sample.toml | save
· ▲
· ╰── missing filename
╰────
help: Usage: save {flags} <filename>
test commands::save::figures_out_intelligently_where_to_write_out_with_metadata ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 515 filtered out; finished in 0.10s
```
