split lexer into parts, move lexer and parser in subdirectories.

build.gradle

src/main/jastadd/parser/Preamble.parser

src/main/jastadd/parser/RelAst.parser

src/main/jastadd/scanner/Header.flex

src/main/jastadd/scanner/Keywords.flex

src/main/jastadd/scanner/Macros.flex

src/main/jastadd/scanner/Preamble.flex

src/main/jastadd/scanner/RulesPostamble.flex

src/main/jastadd/scanner/RulesPreamble.flex

src/main/jastadd/scanner/Symbols.flex