When a literal appears in the file multiple times its identity should be
preserved.
CLHS 3.2.4.4:
> If two literal objects appearing in the source code for a single
> file processed with the file compiler are the identical, the
> corresponding objects in the compiled code must also be the identical.
Previously, every bytecode object created during ext::bc-compile-file
had its own vector of constants making it impossible to satisfy this
constraint. Thus, we change ext::bc-compile-file to use the same
constants vector for all bytecode objects from the same file. The
simplest way to achieve this is to use the same compiler environment
for all of the compilation process and push the read-compile loop
into the si_bc_compile_from_stream function implemented in C.
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