add arg_tuple_expr

[oxinabox]

This is useful to have and something that is needed for Nabla invenia/Nabla.jl#189
(It is extracted out of that PR)

[mzgubic]

i was going to complain it doesn't deal with kwargs, but that seems by design (and maybe addressed by other PRs, haven't yet checked)

only one real comment, the rest is just fixing typos

[oxinabox]

i was going to complain it doesn't deal with kwargs, but that seems by design

This is by design, kwargs are not a feature I need right now.
I will add to the docstring that it only does positional arguments

[codecov]

Codecov Report

Merging #17 (3c00805) into master (6063438) will decrease coverage by 0.44%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   99.42%   98.97%   -0.45%     
==========================================
  Files           4        5       +1     
  Lines         174      196      +22     
==========================================
+ Hits          173      194      +21     
- Misses          1        2       +1     

Impacted Files	Coverage Δ
src/ExprTools.jl	100.00% <ø> (ø)
src/def_tools.jl	95.45% <95.45%> (ø)

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