cleanup: rename name_from to symbol_from
cleanup: rename name_from to symbol_from

Lexer uses Symbols for a lot of stuff, not only for identifiers, so
the "name" terminology is just confusing.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust