Enable shortened/normal mixed record definition

[ytoml]

Fixes #238.

Enable record definition in mixed style:

x = 1
z = 100
t = {x; y = 10; z}
print! t.x  # prints 1
print! t.y  # prints 10
print! t.z  # prints 100

@mtshiba

[mtshiba]

Thank you! I found another problem during the review, so I made a new issue.

240