Remove method ambiguity in isequal(::Symbolic, ::Missing).

[zengmao]

According to the linter Aqua.jl, SymbolicsUtils has method ambiguities associated with isequal and promote_symtype:

julia> Aqua.test_ambiguities(SymbolicUtils)
50 ambiguities found. To get a list, set `broken = false`.
Ambiguity #1
isequal(x, ::SymbolicUtils.Symbolic) @ SymbolicUtils ~/.julia/dev/SymbolicUtils/src/types.jl:227
isequal(::Missing, ::Any) @ Base missing.jl:81

Possible fix, define
  isequal(::Missing, ::SymbolicUtils.Symbolic)

Ambiguity #2
isequal(::SymbolicUtils.Symbolic, x) @ SymbolicUtils ~/.julia/dev/SymbolicUtils/src/types.jl:226
isequal(::Any, ::Missing) @ Base missing.jl:82

Possible fix, define
  isequal(::SymbolicUtils.Symbolic, ::Missing)

# 48 more ambiguities omitted, associated with `promte_symtype`.

This PR removes the ambiguities associated with isequal by defining the specific methods,

Base.isequal(::Missing, ::SymbolicUtils.BasicSymbolic{Real}) = false
Base.isequal(::SymbolicUtils.BasicSymbolic{Real}, ::Missing) = false

[github-actions]

Benchmark Results

	master	0288135...	master/0288135946eb66...
overhead/acrule/a+2	0.725 ± 0.017 μs	0.756 ± 0.02 μs	0.96
overhead/acrule/a+2+b	0.713 ± 0.019 μs	0.726 ± 0.018 μs	0.982
overhead/acrule/a+b	0.248 ± 0.013 μs	0.258 ± 0.013 μs	0.961
overhead/acrule/noop:Int	25.6 ± 0.051 ns	25.9 ± 0.92 ns	0.988
overhead/acrule/noop:Sym	0.0367 ± 0.0057 μs	0.0365 ± 0.0053 μs	1.01
overhead/rule/noop:Int	0.0443 ± 0.00052 μs	0.0443 ± 0.00069 μs	1
overhead/rule/noop:Sym	0.0557 ± 0.0034 μs	0.0554 ± 0.0029 μs	1
overhead/rule/noop:Term	0.055 ± 0.0029 μs	0.0556 ± 0.0028 μs	0.989
overhead/ruleset/noop:Int	0.133 ± 0.0042 μs	0.129 ± 0.0029 μs	1.03
overhead/ruleset/noop:Sym	0.15 ± 0.0059 μs	0.148 ± 0.0047 μs	1.02
overhead/ruleset/noop:Term	3.12 ± 0.14 μs	3.1 ± 0.2 μs	1.01
overhead/simplify/noop:Int	0.137 ± 0.0012 μs	0.147 ± 0.0016 μs	0.933
overhead/simplify/noop:Sym	0.155 ± 0.0057 μs	0.174 ± 0.0059 μs	0.889
overhead/simplify/noop:Term	0.037 ± 0.0022 ms	0.038 ± 0.0026 ms	0.975
overhead/simplify/randterm (+, *):serial	0.09 ± 0.0018 s	0.0909 ± 0.0016 s	0.991
overhead/simplify/randterm (+, *):thread	0.052 ± 0.033 s	0.0533 ± 0.032 s	0.975
overhead/simplify/randterm (/, *):serial	0.216 ± 0.0072 ms	0.221 ± 0.0075 ms	0.977
overhead/simplify/randterm (/, *):thread	0.245 ± 0.0084 ms	0.251 ± 0.0088 ms	0.977
overhead/substitute/a	0.0591 ± 0.0017 ms	0.0618 ± 0.0017 ms	0.958
overhead/substitute/a,b	0.052 ± 0.0016 ms	0.0558 ± 0.0018 ms	0.932
overhead/substitute/a,b,c	17.1 ± 0.67 μs	18.7 ± 0.81 μs	0.915
polyform/easy_iszero	29.3 ± 2 μs	30 ± 1.9 μs	0.977
polyform/isone	2.79 ± 0.01 ns	2.79 ± 0 ns	0.996
polyform/iszero	1.14 ± 0.041 ms	1.16 ± 0.035 ms	0.983
polyform/simplify_fractions	1.66 ± 0.049 ms	1.71 ± 0.045 ms	0.967
time_to_load	2.15 ± 0.0074 s	2.16 ± 0.012 s	0.994

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.86%. Comparing base (65c0293) to head (0288135).
Report is 186 commits behind head on master.

Files	Patch %	Lines
src/types.jl	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #639      +/-   ##
==========================================
- Coverage   83.14%   81.86%   -1.29%     
==========================================
  Files          16       16              
  Lines        1869     1908      +39     
==========================================
+ Hits         1554     1562       +8     
- Misses        315      346      +31     

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