reduce code size in initialization functions (letrec_main)

[gerdstolpmann]

We already prevent local variables from being allocated and write all values to the stack. More is possible:

• Shorter allocation snippets: instead of alloc_fast/alloc_slow just use caml_alloc_small
• Shorter initialization of blocks: e.g. blockinit3(value, offset, p1, p2, p3) to write three values at value[offset], value[offset+1] and value[offset+2] (will need such functions for e.g. n=1 to n=5, and also shr versions that base on caml_initialize)
• Shorter function calls