Use ExprTools, reuse _timer_expr, rework timeit_debug func def.

[maleadt]

In order to add a feature, I wanted to facilitate code reuse between timer_expr and timer_expr_func. That prompted me to use ExprTools to simplify the implementation, which surfaced a bug similar to #65 (comment). Since I wanted to keep the code reuse, I decided to implement #61 (comment).

Fixes #61

[codecov-commenter]

Codecov Report

Merging #117 (ec58e16) into master (90a5ee5) will decrease coverage by 2.78%.
The diff coverage is 92.30%.

❗ Current head ec58e16 differs from pull request most recent head 7591e1a. Consider uploading reports for the commit 7591e1a to get more accurate results

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
- Coverage   93.10%   90.32%   -2.79%     
==========================================
  Files           4        5       +1     
  Lines         377      372       -5     
==========================================
- Hits          351      336      -15     
- Misses         26       36      +10     

Impacted Files	Coverage Δ
src/TimerOutputs.jl	100.00% <ø> (ø)
src/TimerOutput.jl	84.40% <89.28%> (-6.14%)	⬇️
src/precompile.jl	100.00% <100.00%> (ø)
src/utilities.jl	95.23% <0.00%> (-1.59%)	⬇️
src/show.jl	96.22% <0.00%> (+0.89%)	⬆️

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