Check non-alphanumeric variable names and first char for digits

Closes #843

See merge request redox-os/ion!940

 Implement CDPATH

Closes #494

See merge request redox-os/ion!942

Fix error for using statements in aliases

Closes #855

See merge request redox-os/ion!939

Simplify the assignments code

See merge request redox-os/ion!938

Fix Gitlab reports: use upstream master

See merge request redox-os/ion!937

Add benchmark and binary size tests to CI

See merge request redox-os/ion!933

Update README.md

See merge request redox-os/ion!936

Trim start and end of commands

See merge request redox-os/ion!930

Fix ctrl-c with interactive session

See merge request redox-os/ion!932

Require cargo fmt

See merge request redox-os/ion!929

Update to new toolchain

See merge request !928

Fix the benchmark

See merge request !926

Move the test to its own file
Copy 3 times the same program to better evaluate efficiency
Don't use `println`: buffering skews the benchmark

Terminator iterator

See merge request redox-os/ion!922

SmallVec is not included because benchmarks showed 20% regression.
Terminator can't be reset because that also made the code slower.