Fix premature lowering of `K"core_@cmd"` etc

[c42f]

There are four macrocalls to special Kinds which are arguably premature lowering.

K"core_@cmd"
K"core_@int128_str"
K"core_@uint128_str"
K"core_@big_str"

In the same way that we removed K"core_@doc previously (see #217), these should have their own kinds instead:

K"cmdstring" # ??
K"Int128"
K"UInt128"
K"BigInt"

For K"core_@cmd" there might be some thought required: it could be either

• A single K"CmdString", with the delimiters as separate trivia tokens in the parent?
• A K"cmdstring" container - in analogy to K"string" - with a K"String" or K"CmdString" chunk inside?