Add support for anonymous functions in excellent

[rowanseymour]

Currently support just named functions, e.g.

@(foreach(array("a", "b", "c"), upper)) -> [A, B, C]
@(foreach(array("the man", "fox", "jumped up"), word, 0)) -> [the, fox, jumped]

But this adds syntax for anonymous functions (aka lambdas #768), e.g.

@(foreach(array("a", "b", "c"), (x) => x & "1")) -> [a1, b1, c1]
@(foreach(array("a", "b", "c"), (x) => object("v", x))) -> [{v: a}, {v: b}, {v: c}]

Will be helpful for nyaruka/courier#380

[codecov]

Codecov Report

Merging #1063 (837b308) into excellent3 (7330215) will increase coverage by 0.00%.
The diff coverage is 92.10%.

@@             Coverage Diff             @@
##           excellent3    #1063   +/-   ##
===========================================
  Coverage       86.63%   86.64%           
===========================================
  Files             256      256           
  Lines           10696    10720   +24     
===========================================
+ Hits             9267     9288   +21     
- Misses           1002     1004    +2     
- Partials          427      428    +1     

Impacted Files	Coverage Δ
excellent/functions/builtin.go	97.94% <ø> (ø)
excellent/types/function.go	68.75% <83.33%> (-0.49%)	⬇️
excellent/evaluate.go	89.65% <84.61%> (-2.35%)	⬇️
excellent/base.go	100.00% <100.00%> (ø)
excellent/tree.go	100.00% <100.00%> (ø)
excellent/visitor.go	93.33% <100.00%> (+0.56%)	⬆️
flows/runs/run.go	80.08% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7330215...837b308. Read the comment docs.