Fix macro hygiene in @constant, @derived_constant (missing esc())

[Socob]

The constant definition macros have some issues where not all inputs are escaped using esc(), so that existing variables cannot be referred to in some of the arguments. For example, this affects the val argument of @constant and @derived_constant (but not the corresponding def). I suspect this is why derived constants have their Float64 values (but not their BigFloat values!) hard-coded using float literals instead of referring to previously defined constants, e. g.

PhysicalConstants.jl/src/codata2018.jl

Lines 47 to 48 in 7d94def

	@derived_constant(ReducedPlanckConstant, ħ, "Reduced Planck constant",
	1.0545718176461565e-34, ustrip(big(h))/(2 * big(pi)), J * s,

PhysicalConstants.jl/src/codata2018.jl

Lines 80 to 81 in 7d94def

	@derived_constant(MolarGasConstant, R, "Molar gas constant", 8.314_462_618_153_24,
	ustrip(big(N_A) * big(k_B)), J * mol^-1 * K^-1,

PhysicalConstants.jl/src/codata2018.jl

Lines 87 to 88 in 7d94def

	@derived_constant(StefanBoltzmannConstant, σ, "Stefan-Boltzmann constant", 5.670_374_419_184_4294e-8,
	ustrip(2 * big(pi) ^ 5 * big(k_B) ^ 4 / (15 * big(h) ^ 3 * big(c_0) ^ 2)), W * m^-2 * K^-4,

This PR fixes the macro hygiene issues and changes these constant definitions to avoid unnecessary hard-coded values. I’ve checked that the resulting Float64 values are the same as those that were previously hard-coded.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.75%. Comparing base (7d94def) to head (1b7d302).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   93.50%   93.75%   +0.24%     
==========================================
  Files           3        3              
  Lines          77       80       +3     
==========================================
+ Hits           72       75       +3     
  Misses          5        5              

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[giordano]

Thanks!