Add source amalgamation

The amalgamated source concatenates all sources
to a file janet.c which can be used for
embedding janet, much in the same way as sqlite
or mongoose.

src/core/emit.h

@@ -23,7 +23,9 @@
 #ifndef JANET_EMIT_H
 #define JANET_EMIT_H
 
+#ifndef JANET_AMALG
 #include "compile.h"
+#endif
 
 void janetc_emit(JanetCompiler *c, uint32_t instr);