Try u_str without eval

[tshort]

This is to help with precompiling (#133). It's generally frowned upon to use eval in a macro. This is an attempt to return the expression rather than evaling it. The main difference is that u"N*m" resolves to Unitful.N * Unitful.m whereas now, it resolves to the result of that multiplication.

Making u_str work with precompilation will make this macro useful in other packages (most other packages are going to want precompilation).

I tested this in a small package, and it does get rid of the eval warnings related to precompilation.

[codecov-io]

Codecov Report

Merging #161 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   97.53%   97.52%   -0.02%     
==========================================
  Files          15       15              
  Lines         689      686       -3     
==========================================
- Hits          672      669       -3     
  Misses         17       17

Impacted Files	Coverage Δ
src/user.jl	97.11% <100%> (ø)	⬆️
src/units.jl	94.5% <0%> (-0.06%)	⬇️
src/quantities.jl	97.03% <0%> (-0.03%)	⬇️
src/range.jl	100% <0%> (ø)	⬆️
src/promotion.jl	100% <0%> (ø)	⬆️

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[ajkeller34]

Sorry for the delay, I think this is a reasonable approach (I rebased and force pushed).