Library feature

[minghui-liu]

Hi. This is my implementation of the libraries feature. Suggestions are welcome.

Here is brief summary of changes:

• added a new structure Lib that contains a list of symbols and a list of Defns to represent a library.
• added a new instruction for global and the matching code in printer.rkt to print its assembly.
• read in compile-file.rkt was replaced by read-sequence to read a list of forms
• check-label-targets-declared was removed because now we can have labels declared outside of current file. handle-undeclared-labels was added to list these undeclared labels using extern
• asm-string was modified so that it does not try to find the first label and prepend a global in the case of a library, determined by if the list of instructions already begins with a Global (added by compiling the provide clause)

Two files, length.rkt and use-le.rkt were included to demo the feature.

make length.o
make use-len.o
ld -r main.o char.o io.o length.o -o runtime.o
gcc runtime.o use-len.o -o use-len
./use-len

Some drawbacks of this implementation:

• Two tests were broken because of the changes made to parse to accommodate read-sequence and need to be fixed
• globals are not at the top of the assembly file
• the way to distinguish between a program and library (by the first instruction in the sequence) is hacky

[minghui-liu]

Closed because #35 was merged.