Better record spread.

[toots]

This PR improves record spread by allowing multiple spread inside records:

  x = {foo=123}
  y = {gni="aabb"}
  test.equals({...x, ...y}, {foo=123, gni="aabb"})
  test.equals({...x, gna=3.14, ...y}, {foo=123, bla=3.14, gni="aabb"})
  test.equals(
    {...x, gna=3.14, ...y, foo="bar"}, {foo="bar", bla=3.14, gni="aabb"}
  )

Refs: #3512

[vitoyucepi]

Hi @toots,
I think you should add more tests.

x = {foo=1}
y = {foo=2}
test.equals({...x, ...y}, {foo=2})
test.equals({...y, ...x}, {foo=1})

x = {foo=1}
y = {foo=null()}
test.equals({...x, ...y}, {foo=null()})
test.equals({...y, ...x}, {foo=1})

x = {foo=null()}
y = {foo=null()}
test.equals({...x, ...y}, {foo=null()})
test.equals({...y, ...x}, {foo=null()})

x = {foo=1}
y = {foo="a"}
test.equals({...x, ...y}, {foo="a"})
test.equals({...y, ...x}, {foo=1})

x = {foo=1}
y = {foo={bar=1}}
test.equals({...x, ...y}, {foo={bar=1}})
test.equals({...y, ...x}, {foo=1})

[toots]

x = {foo=1}
y = {foo=2}
test.equals({...x, ...y}, {foo=2})
test.equals({...y, ...x}, {foo=1})

x = {foo=1}
y = {foo=null()}
test.equals({...x, ...y}, {foo=null()})
test.equals({...y, ...x}, {foo=1})

x = {foo=null()}
y = {foo=null()}
test.equals({...x, ...y}, {foo=null()})
test.equals({...y, ...x}, {foo=null()})

x = {foo=1}
y = {foo="a"}
test.equals({...x, ...y}, {foo="a"})
test.equals({...y, ...x}, {foo=1})

x = {foo=1}
y = {foo={bar=1}}
test.equals({...x, ...y}, {foo={bar=1}})
test.equals({...y, ...x}, {foo=1})

Added thanks!