~emersion/mrsh

ref: d4e63a2bd24c9b31a9a44e182a33978d787f8464 mrsh/test/arithm.sh -rw-r--r-- 881 bytes
shell/arithm: implement assignments

The parser still chokes on assignments with an operation.
Partially implement arithmetic expansion

The only expansion missing is assignment expansion, which requires more
discussion.

Additionally, the parser is missing variable support, e.g. $((var+1)).