Many small changes to shell code generation

[laurenthuberdeau]

Context

This PR is best reviewed commit-by-commit.

As we write the article, we'll want to make small changes to the shell code generation. Notable changes are:

• Rename alphanumerical char constants to _{char}_
• Remove leading and trailing whitespace in $(( ... ))
• Allow local variables to be called argv
• Replace alloc with _malloc
• Remove redundant parenthesis in arithmetic expressions
• Define function parameters using let and move let call on same line as shell function declaration.