Add lambdas, add chaining, fix docstrings

[THinnerichs]

I got a bit carried away, so this includes way more things than I originallly anticipated:

• Added lambda function support

g = @csgrammar begin
                Arr = _arg_1
                Arr = map(Func, Arr)

                Func = inc
                Func = x -> 2*x
                Func = x -> x + 1
                Func = x -> 2*Func(x) + 1
                Func = x -> Func(Func(x))
            end

            interpret_custom = HerbInterpret.make_interpreter(
                g;
                target_module=@__MODULE__,
            )

            input = Dict{Symbol,Any}(:_arg_1 => [1, 2, 3, 4])
            
            # map(x -> (x -> 2*x)(inc(x)), arr) = map(x -> 2*inc(x), arr)
            interpret_custom(@rulenode(2{7{4,3},1}), input) # yiels [4, 6, 8, 10]

• added support for chaining of operators in stateful grammars:

module LocalStateDSL2
    using HerbCore
    using HerbGrammar
    using RuntimeGeneratedFunctions
    RuntimeGeneratedFunctions.init(LocalStateDSL2)
 
    struct St
        x::Int
    end

    Base.:(==)(a::St, b::St) = a.x == b.x

    inc(st::St) = St(st.x + 1)
    dec(st::St) = St(st.x - 1)
    iseven(st::St) = Base.iseven(st.x)

    g2 = @cfgrammar begin
        Start = Step
        Step  = IF(Cond, Step, Step)
        Step  = inc()
        Step  = dec()
        Step  = (Step; Step)
        Cond  = iseven()
    end
end

interp2 = HerbInterpret.make_stateful_interpreter(
                LocalStateDSL2.g2;
                target_module = LocalStateDSL2,
                cache_module  = @__MODULE__,
            )
            
prog_two_incs = @rulenode(5{3,3})
interp2(prog_two_incs, LocalStateDSL2.St(0)) # yields LocalStateDSL2.St(2)

• added tests
• updated docstrings for core functions

[codecov]

Codecov Report

❌ Patch coverage is 81.11111% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.21%. Comparing base (b5a6c5c) to head (2f64610).

Files with missing lines	Patch %	Lines
src/make_interpret.jl	81.11%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   69.11%   74.21%   +5.10%     
==========================================
  Files           3        3              
  Lines         204      256      +52     
==========================================
+ Hits          141      190      +49     
- Misses         63       66       +3     

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[github-actions]

Benchmark Results

	master	2f64610...	master / 2f64610...
interpret/Random Expressions	0.123 ± 0.0016 ms	0.123 ± 0.0016 ms	0.999 ± 0.019
time_to_load	0.0847 ± 0.0022 s	0.0863 ± 0.0015 s	0.981 ± 0.031

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).