run Aboriginal Linux correctly about oil

[andychu]

Implement Slicing of arrays and strings:

77e8e89

Top-level return (e.g. it was used in subshells in Aboriginal Linux)

6841a09

unset R of readonly variable should return 1, not abort the script:

f9a5986

Implement echo -e

9c8814d

Fix && and || precedence:

100d2e6

Fix FOO=bar myfunc

670276e

Tracing improvements, which helped me find the lines of code that were misinterpreted:

Implement $SHELLOPTS

934deba

Basic xtrace that evaluates $PS4:

9887c70

$SOURCE_NAME and $LINENO

3d6099a

[andychu]

Note on MANY examples of divergent behavior

• When FOO=bar can refer to previous bindings
• What backslash escapes are accepted read (no -r)
• mksh: doesn't respect nounset with patsub
• top level return

etc. Essentially every feature exposed some shell incompatibility.

[andychu]

Also: read -r and read respecting IFS.

[andychu]

Done for now, needs more automation and regular testing.