Lexer bug: Parsing of TyVariants

[gizemcaylak]

After adding type matching support in boot, test cases related to TyVariants have failed in stdlib/mexpr/boot-parser.mc.
because TyVariants are not parsed correctly. For example, after parsing this statement

type Foo=<> in x

< and > have variable types with idents < and > respectively. I guess the support for the syntax should be added in the lexer.